Composite solar-control coating based on tungsten bronze nanocrystals dispersed in a silica-based sol-gel matrix

Abstract

The invention relates to a sol formulation which can be used to form a solar-control coating, in particular a coating that blocks UV and NIR radiation, comprising at least: one or more silica-based sol-gel matrix precursors, and doped tungsten bronze nanocrystals which are dispersed uniformly and individually in a protic solvent medium. The invention also relates to a method for forming a solar-control coating at the surface of a support using such a sol formulation, and also to a structure comprising at least one support, preferably a transparent support, having such a solar-control coating on at least one face thereof.

Description

[0001] Description

[0002] Title: Composite solar control coating based on tungsten bronze nanocrystals dispersed in a silica-based sol-gel matrix

[0003] Technical field

[0004] The present invention relates to the field of developing coatings for solar control, making it possible to effectively filter the different ranges of solar radiation. It relates more particularly to the formation of a coating, based on tungsten bronze type plasmonic nanocrystals, having improved optical performance, in particular making it possible to effectively block ultraviolet and / or near infrared rays. These coatings can find various applications, for example being used for glazing.

[0005] Prior art

[0006] The development of glazing which, while remaining transparent, has protective properties against ultraviolet radiation, referred to as UV in the rest of the text, and / or near infrared radiation, also known as NIR (for "Near InfraRed" in Anglo-Saxon terminology), is attracting growing interest for a wide range of applications, in particular in the manufacture of windows for buildings, for vehicles, greenhouses in the agricultural sector, etc.

[0007] In fact, among the radiation contained in sunlight, ultraviolet (UV) rays are undesirable, being likely to cause damage to human skin, accessories or equipment inside or on board vehicles, while near infrared (NIR) rays cause a significant increase in temperature inside.

[0008] In particular, in the current context where the energy renovation of buildings is becoming an environmental and technological issue, the development of solar control glazing, i.e. glazing that filters the different wavelength ranges of solar radiation, appears to be an important technological lever for limiting energy consumption for heating or air conditioning. Thus, when it is cold, it is important to be able to keep the heat created by heating appliances inside a room or vehicle, generally located in the mid-infrared (MIR) range between 3 and 18 pm [1]. On the other hand, when it is hot, it is a question of being able to block outside the near-infrared radiation of solar emission, located in the wavelength range from 780 to 2500 nm, and representing 50% of solar radiation.

[0009] Thus, it is desirable that the glazing not only has good transparency properties, in other words at least partial transmission of radiation in the visible range (noted Vis) of sunlight, but also that it can block the ultraviolet and near infrared radiation ranges, in order to protect and thermally insulate the interior.

[0010] Several coating technologies for glazing, e.g., windows, windshields, conservatories, greenhouses, have already been proposed to block ultraviolet and / or near-infrared radiation. The most commonly used thermal screens are based on metallic layers or low-emissivity coatings that effectively reflect MIR [2]. However, to effectively block NIR, it is necessary to implement complex stacking structures of several functional layers.These approaches unfortunately come at a high cost, typically at least 10 times higher than the cost of uncoated glass, and generally achieve selectivity in terms of wavelength transmission / extinction (which can be assessed by the Tvis:Tuv and Tvis:TNIR ratios, ratios between the transmission in the visible wavelength range (Tvis) and the transmission in the UV and NIR wavelength range respectively (Tuv and TNIR)).

[0011] So-called "plasmonic" particles, which exhibit strong absorption for a small amount of material (high optical density) and very high selectivity in their absorption wavelength range, are emerging as a promising solution for protection against NIR.

[0012] Traditional metals, such as silver or gold, have a high density of free carriers (10 22 cm' 3) which, when confined at the nanoscale, drive the collective oscillation of their free electrons, called localized surface plasmon resonance (LSPR). The phenomenon of metallic LSPR has been widely studied in recent decades and finds applications in a wide variety of optical domains ([3]). However, the range of their absorption is limited to the ultraviolet and visible, leaving the NIR range mostly inaccessible for metals, except for complex architectures such as core-shell and highly anisotropic nanofds ([4]).

[0013] In this context, highly doped semiconductor nanocrystals are attracting increasing interest as absorbing materials [5]. Their free carrier density can be tuned from 10 18 at 10 22 cm' 3by modifying their doping level ([6]), directly during their synthesis ([7],) or by subsequent treatments (post-treatments) ([8]). This tool, added to the known parameters for metals such as shape, size or surrounding environments, and to the numerous composition possibilities, allows extremely precise control of the position of their LSPR, from visible to mid-infrared (MIR) [9]. As a result, their unique characteristics are exploited for many applications, for example for LSPR detection (

[0010] ); in bio-imaging and therapy (

[0011] ) and the development of smart windows

[0012] . In particular, semiconductor nanocrystals, allowing strong absorption in the NIR, while maintaining transparency in the visible range, thus appear as good candidates for obtaining coatings for solar control.

[0014] In particular, many studies use nanoparticles of indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO), to produce active or passive devices (

[0013] ). However, their LSPR, located in the wavelength region of 1500-2500 nm, leaves a high transmission of radiation in the wavelength range of 780 to 1500 nm.

[0015] More recently, the development of new nanocrystalline compositions, such as rare earth hexaborides (RBÔ, R=La, Ce, Pr, Nd, Gd) (

[0014] ,) and tungsten bronzes (M X W03, M=K, Na, Cs) (

[0015] ,

[0016] ,

[0017] ,

[0018] ,

[0019] ) have made it possible to achieve LSPR, and therefore absorption of radiation, in the NIR wavelength range.

[0016] Unfortunately, the incorporation of these nanocrystals into solid thin films, without degradation of the intensity and selectivity of their LSPR, remains a challenge. Indeed, in the context of the implementation of these nanocrystals in conventional processes for forming surface coatings, a coupling effect linked to the connection of the nanocrystals to each other at the level of the compact structure of the film, causes a decrease in the intensity of the LSPR and a red shift, while the aggregation of the nanocrystals within the formed coatings leads to an increase in scattering in the visible range (

[0020] ).

[0017] Among the methods proposed for the preparation of coatings based on semiconductor nanocrystals, we can cite the publication of Zeng et al.

[0021] which describes the fabrication of composite resin thin films incorporating Cso,33W03 nanoparticles prepared by bulk milling methods, which provides limited control over their sizes and shapes.

[0018] One can also cite the publication by T. Mattox et al.

[0014] which proposes a way to disperse colloidal LaBô in a polymer or sol-gel silica matrix by incorporating ligands during the synthesis via sodium borohydride solvent. However, this methodology cannot be applied to most plasmonic metal oxide semiconductors whose synthesis routes use organometallic complex precursors that react at high temperature in non-polar solvents. By modifying various parameters, these syntheses allow precise control of the size and shape of the particles, but leave them capped with non-polar ligands, which makes their dispersion in most polymer or silica media difficult.

[0019] There thus remains a need to have a means of taking advantage of the advantageous optical properties of tungsten bronze type plasmonic nanocrystals when implementing them in a surface coating.

[0020] More particularly, there remains a need for a method for developing a coating based on tungsten bronze type plasmonic nanocrystals, without hindering the properties of these nanocrystals, in particular by maintaining high extinction of radiation in the UV and NIR ranges, while maintaining good transparency in the visible range.

[0021] The invention aims precisely to meet these needs.

[0022] Statement of the invention

[0023] The present invention thus provides a means of accessing a thin film type coating based on doped tungsten bronze nanocrystals, without degrading the intrinsic properties of these nanocrystals, in particular in terms of intensity and selectivity of their LSPR, and therefore making it possible to form a coating for solar control, exhibiting high extinction of radiation in the UV and NIR ranges, while maintaining high transparency in the visible range.

[0024] More particularly, the inventors have discovered that it is possible to obtain a coating having the required optical properties, in particular capable of effectively blocking UV and near-infrared radiation, by dispersing doped tungsten bronze nanocrystals, preferably of controlled morphology and size, in a homogeneous and individualized manner within a silica-based sol-gel matrix.

[0025] Such a composite coating for solar control may be more particularly prepared from a sol formulation comprising a mixture of one or more precursors of said silica-based sol-gel matrix and said doped tungsten bronze nanocrystals, dispersed in a homogeneous and individualized manner in a protic solvent medium.

[0026] Thus, the present invention relates, according to a first of its aspects, to a sol formulation, useful for forming a solar control coating, in particular a coating blocking UV and NIR radiation, said sol formulation comprising at least:

[0027] - one or more silica-based sol-gel matrix precursors, and

[0028] - nanocrystals of type MxWOa-y, with M representing potassium (K), sodium (Na) or cesium (Cs), x ranging from 0.05 to 0.33 and y ranging from 0 to 0.4, called doped tungsten bronze nanocrystals, dispersed in a homogeneous and individual manner in a protic solvent medium.

[0029] Preferably, as detailed in the rest of the text, the doped tungsten bronze nanocrystals advantageously have adequate surface functionalization, so as to promote their dispersion within said soil formulation and within the composite coating formed from this formulation.

[0030] The nanocrystals may preferably be functionalized by at least one ligand capable of promoting good dispersion of said nanocrystals within the sol formulation.

[0031] Such ligands may be, for example, ligands bearing hydroxyl functions, for example polyglycerol ligands, in particular of the hyperbranched (or hyper-ramified) polyglycerol type, or organo-functional polyphosphate or silane ligands, such as gamma-glycidoxypropyltrimethoxysilane (GLYMO) and (3-aminopropyl)triethoxysilane (APTES), in particular said ligand(s) being able to be hyper-ramified polyglycerols. The invention also relates, according to another of its aspects, to the use of a sol formulation according to the invention for forming a solar control coating, in particular blocking UV and NIR radiation, on the surface of a support, in particular on the surface of a transparent support and more particularly of a support made of glass or transparent polymer(s).

[0032] In particular, the invention relates to a method for forming a solar control coating, in particular blocking UV and NIR radiation, on the surface of a support, in particular on the surface of a support made of glass or transparent polymer(s), comprising at least the steps consisting of:

[0033] (i) having a sol formulation according to the invention, comprising one or more silica-based sol-gel matrix precursors and doped tungsten bronze nanocrystals, preferably surface-functionalized by at least one ligand as defined above, in particular ligands carrying hydroxyl functions, said nanocrystals being dispersed in a homogeneous and individualized manner in a protic solvent medium;

[0034] (ii) depositing a layer of said soil formulation on the surface of said support; and

[0035] (iii) drying the layer formed in step (ii) so as to obtain said silica-based sol-gel matrix.

[0036] The invention also relates, according to another of its aspects, to a structure comprising at least one support, preferably transparent, in particular made of glass or transparent polymer(s), having on at least one of its faces a solar control coating, in particular blocking UV and NIR radiation, formed from a sol formulation according to the invention as defined above, and more particularly via a method according to the invention as defined above. Such a coating more particularly comprises a silica-based sol-gel matrix in which doped tungsten bronze nanocrystals are dispersed in a homogeneous and individual manner.

[0037] In the context of the present invention, the term “individually” or “individually” dispersed nanocrystals, or “individualized” nanocrystals, within a given medium, for example within a soil formulation or a coating, is understood to mean that the nanocrystals are not aggregated, in other words do not appear in the form of aggregates. In particular, the distance between two individualized nanocrystals is strictly greater than their largest dimension, in particular at least once greater than their largest dimension.

[0038] The set of nanocrystals considered according to the invention (at the level of the dispersion or the coating formed) may possibly contain nanocrystals not meeting this characteristic, insofar as the non-aggregation criterion is respected by at least 60% by number, in particular at least 70% by number of the nanocrystals of the set. Preferably, at least 80%, in particular at least 90%, preferably at least 95% by number of the nanocrystals of the set considered are individualized.

[0039] By "homogeneous" is meant that the nanocrystals are distributed uniformly in the volume of the dispersion or coating formed, on the scale of a hundred nanometers. The homogeneity of the dispersion of the nanocrystals within the coating formed according to the invention can be evaluated as detailed in the examples which follow, by analysis of the images obtained by 3D tomographic transmission electron microscopy, in particular using the Voronoi cell algorithm.

[0040] As illustrated in the following examples, the inventors have found that the intrinsic optical properties of the doped tungsten bronze nanocrystals, particularly in terms of the intensity and position of their LSPR, are advantageously preserved within the hard and protective silica-based sol-gel matrix.

[0041] Advantageously, it is thus possible to easily modulate the optical properties of the coating, in particular the extinction selectivity in the UV and NIR ranges, by controlling the composition of the nanocrystals used, in particular their doping rate, their size and their morphology.

[0042] In fact, as previously indicated, doped tungsten bronze nanocrystals can simultaneously exhibit a localized surface plasmon resonance called LSPR, strongly absorbing NIR radiation, as well as a strong absorption in UV radiation thanks to their band gap energy located at the border between the visible and the UV.

[0043] Ultraviolet radiation refers to the part of the electromagnetic spectrum in the wavelength range from 200 nm to 390 nm, while near-infrared radiation refers to the part of the electromagnetic spectrum in the wavelength range from 780 nm to 2500 nm. The visible spectrum refers to the part of the electromagnetic spectrum in the wavelength range from 390 nm to 780 nm.

[0044] Advantageously, the doped tungsten bronze nanocrystals used have a controlled size and morphology so as to adjust the spectral position of their localized surface plasmon resonance (LSPR) peak, and thus their NIR absorption selectivity.

[0045] Also, it is possible to vary the alkali doping rate of the tungsten bronze nanocrystals used, so as to control the position of the absorption by the forbidden band in the vicinity of the UV.

[0046] Thus, it is possible to access a coating with optimized optical properties, in particular effectively blocking UV and NIR radiation, by adjusting the intrinsic properties of the nanocrystals used. The ability of the coating to act as a screen against UV and NIR radiation can be evaluated, as detailed in the examples that follow, by measuring the extinction spectra of the fdms, for example using a spectrophotometer.

[0047] In particular, the coating may have a percentage of NIR absorption, noted ANIR, greater than or equal to 60%, in particular greater than or equal to 70%, and / or a selectivity of transmission of solar energy, known as “SETS”, greater than or equal to 0.70, in particular greater than or equal to 0.75.

[0048] The selectivity of transmission of solar energy, called "SETS" (for "Solar energy transmittance selectivity" in Anglo-Saxon terminology)

[0018] , as well as the percentage of absorption of NIR, noted ANIR, can be calculated from the convolution of the extinction spectra of a film with solar radiation, for a transmittance with respect to the visible fixed at 80%.

[0049] In particular, the percentage of NIR absorption is defined as follows

[0029] : 100 with INC representing the irradiance after filtering through a medium containing the nanocrystals, in particular through the coating, I soiar representing the irradiance of the sun, and 2 representing the wavelength.

[0050] In particular, the transmission selectivity of solar energy is defined as follows

[0029] : with INC, Isoiar and z being as defined above.

[0051] Furthermore, the coating has high transparency and aesthetically favorable coloring in the visible range. In particular, the coating formed advantageously has a transmittance, over the entire visible spectrum, greater than or equal to 70%, in particular greater than or equal to 80%, in particular greater than or equal to 90% and more particularly greater than or equal to 95%.

[0052] Transmittance represents the light intensity passing through the coating in the visible spectrum. It can be measured, for example, by UV-Vis spectrometry, for example using a Shimadzu UV-3100 spectrometer.

[0053] Furthermore, advantageously, the coating obtained according to the invention has a dispersion of doped tungsten bronze nanocrystals within the silica-based sol-gel matrix, which is homogeneous and individualized, even for a high volume fraction of nanocrystals, in particular up to 20%.

[0054] It is thus possible to access a coating with optimized solar control properties, in particular shielding both UV and NIR radiation, while maintaining good transparency in the visible range.

[0055] The invention thus relates, according to another of its aspects, to the use of a soil formulation according to the invention to give a support solar control properties, in particular to screen against UV and NIR radiation.

[0056] Furthermore, the formation of a coating according to the invention proves to be easy and inexpensive, particularly compared to the coating technologies proposed until now, as discussed above, based on complex stacks of metal layers. In fact, the coating according to the invention can be produced by conventional liquid phase deposition techniques, for example by spin-coating.

[0057] Finally, the composite coating formed according to the invention, in particular in the form of a thin film, in particular with a thickness of less than 3 μm, has good mechanical properties, in particular in terms of film flexibility and resistance to fracturing. Also, the coating formed according to the invention, based on inorganic and chemically / thermally stable oxides, has good durability, in particular durability greater than that of the coatings already proposed in the prior art based on organic polymers or metal deposits.

[0058] The coatings formed according to the invention, having optimized solar control properties, can find multiple applications. They can be implemented for example for glazing, for example for windows of a building, in particular to reduce energy consumption in eco-buildings, for vehicle glazing, for example automobiles, for greenhouses for agriculture, for technical glass, etc. The invention thus relates, according to another of its aspects, to an article comprising at least one structure as defined above, said article being in particular a glazing, for example for windows of a building, verandas, portholes, windshields of automobile-type vehicles, train glazing, greenhouses used in agriculture or even photovoltaic panels.

[0059] Other characteristics, variants and advantages of a coating according to the invention, its preparation and its properties, will become more apparent upon reading the description, examples and figures which follow, given for illustrative and non-limiting purposes of the invention. In the remainder of the text, the expressions "between ... and ...", "ranging from ... to ..." and "varying from ... to ..." are equivalent and are intended to mean that the limits are included, unless otherwise stated.

[0060] Brief description of the drawings

[0061] [Fig la] presents the morphology of the nanocrystals synthesized in example 1.1, exhibiting different form factors (figure a) to d)) and the associated UV-vis-NIR extinction spectra (figure e)).

[0062] [Fig 1b] is a transmission electron microscopy (TEM) photograph of the CsxWCL-y nanocrystals synthesized according to Example 1.2 (Figure 1b(i)); and the histogram of the nanocrystal size distribution (Figure 1b(ii));

[0063] [Fig 2] is the X-ray diffraction (XRD) spectrum of the nanocrystals synthesized in Example 1.2 and in gray the reference spectrum of Cso,32W03 (JCPDS 1524692);

[0064] RECTIFIED SHEET (RULE 91) ISA / EP [Fig 3] shows the extinction spectra of nanocrystals, according to Example 2, in an aggregation state when surface functionalized with oleic acid or in a dispersed state when functionalized with DMOAP, at the same concentration;

[0065] [Fig 4] shows the SEM image of the cross section of the NC@DMOAP film obtained according to Example 2 on a silicon substrate;

[0066] [Fig 5] shows the evolution of the extinction coefficient (in pm' 1 ) and irradiance (W.nT 2 .nm) nanocrystals dispersed in solution and in the film prepared in example 2, and their convolution with solar emission;

[0067] [Fig 6] shows the extinction, reflectance and absorbance spectra of the film prepared in Example 2;

[0068] [Fig 7] schematically represents the preparation of nanocrystals functionalized on the surface by polyglycerol and their implementation in a silica sol-gel matrix, as described in example 3;

[0069] [Fig 8] presents the Fourier transform infrared spectroscopy (FTIR) spectra of the nanocrystals after synthesis according to example 1.2, after surface functionalization by polyglycerol ligands according to example 3 and after insertion into a TMOS:MTMOS matrix;

[0070] [Fig 9] presents the thermogravimetric analysis of nanocrystals functionalized on the surface by polyglycerol, synthesized in example 3;

[0071] [Fig 10] shows a photograph of the solution sample of the surface-functionalized nanocrystals with polyglycerol prepared in Example 3 dispersed in methanol;

[0072] [Fig 11] represents a TEM photograph of the solution of the surface-functionalized nanocrystals with polyglycerol prepared in Example 3 dispersed in methanol (Figure 11(a)) and the histogram of the size distribution of the nanocrystals (Figure 11(b));

[0073] [Fig 12] shows the TEM projections at an angle of 0°C of the films formed in Example 4 reduced by FIB with nanocrystal volume fractions of 2.7% (Figure 12(a)) and 1.1% (Figure 12(b)). The boxes show the Fast Fourier Transform (FFT);

[0074] [Fig 13] presents the 3D modeling of nanocrystals in the silica matrix, with nanocrystal volume fractions of 2.7% (Figure 13(a)) and 1.1% (Figure 13(b)); [Fig 14] presents the comparison of the distribution of the number of nearest neighbors of each nanocrystal, noted NNeighbors between the experiments and the simulations, for the composite coatings formed in Example 4 with different volume fractions of nanocrystals;

[0075] [Fig 15] presents the comparison of the distance between the nearest neighbors, noted dN-N between the experiment and the simulation, for the composite coatings formed in example 4 with different volume fractions of nanocrystals;

[0076] [Fig 16] presents the normalized UV-vis-NIR extinction spectra of composite thin films with different nanocrystal volume fractions in the composite coatings;

[0077] [Fig 17] presents the value of absorbance divided by the optical path as a function of the volume fraction of nanocrystals in the composite coatings; and

[0078] [Fig 18] shows the optical properties of a 6.0 pm thick coating with a nanocrystal volume fraction of 1.1%.

[0079] [Fig 19] shows the extinction spectrum over time of a coating covered with a protective layer, prepared in example 6.

[0080] Detailed description

[0081] DOPED TUNGSTEN BRONZE NANOCRYSTAL

[0082] The nanocrystals used according to the invention are doped metal oxide nanocrystals, known as doped tungsten bronzes, of type M. x WÛ3- y , with M representing potassium (K), sodium (Na) or cesium (Cs), x ranging from 0.05 to 0.33 and y ranging from 0 to 0.4.

[0083] It is understood that the coating formed according to the invention may use a single type of nanocrystals or a mixture of at least two different nanocrystals.

[0084] In a particular embodiment, the nanocrystals are cesium-doped tungsten bronze nanocrystals, in other words nanocrystals of the above-mentioned formula in which M is cesium.

[0085] Advantageously, the nanocrystals are monocrystalline.

[0086] Advantageously, as mentioned previously, the characteristics of M nanocrystals x WÛ3- y , particularly in terms of composition, including free charge carrier density, morphology and size, are controlled and adjusted to achieve the desired UV and NIR absorption selectivities.

[0087] In particular, the alkali metal doping rate, for example cesium, of the doped tungsten bronze nanocrystals used according to the invention can be adjusted with regard to the desired position of the forbidden band in the UV, to obtain the desired absorption selectivity of the wavelengths in the UV.

[0088] In particular, the alkali metal doping rate, particularly caesium, may be between 0.05 and 0.33 and / or the free carrier density may be between 1.10 18 and 9.10 22 cm' 3 .

[0089] Advantageously, the nanocrystals have a controlled morphology, in particular a controlled shape and size, so as to adjust the spectral position of their localized surface plasmon resonance (LSPR) peak, and obtain optimized absorption selectivity of wavelengths in the NIR.

[0090] Preferably, they are in the form of nano-rods. They may have a form factor, noted AR (for "Aspect Ratio" in English terminology, defined as the ratio of the largest dimension of the particle to its smallest dimension), between 0.1 and 20, in particular between 0.4 and 12. In particular, they may have a form factor between 1.2 and 20, more particularly between 1.2 and 3.5, in particular between 1.5 and 2.5.

[0091] They have a length preferably between 4 nm and 100 nm and a width preferably between 5 nm and 100 nm, in particular between 5 nm and 30 nm.

[0092] The size of nanocrystals can be assessed by transmission electron microscopy or by X-ray diffraction analysis. As a note, the characterization of the largest dimension of nanocrystals is most often favored in the direction of crystal growth, particularly along the (001) direction of the crystal.

[0093] In particular, nanocrystals can exhibit a morphology of hexagonal prisms.

[0094] Synthesis of nanocrystals

[0095] Nanocrystals can be synthesized by synthesis routes known to those skilled in the art. Advantageously, the nanocrystals are obtained by a so-called "bottom-up" synthesis route. "Bottom-up" synthesis methods are chemical synthesis methods that rely on the assembly of small chemical entities (atoms or molecules) to produce larger objects, nanocrystals in this case. This type of synthesis is thus distinguished from methods for obtaining nanocrystals by powder grinding.

[0096] Advantageously, bottom-up nanocrystal synthesis allows for precise control of nanocrystal size and morphology.

[0097] More particularly, doped tungsten bronze nanocrystals can be obtained by synthesis, in a solvent medium, from precursors, in particular from tungsten hexacarbonyl (W(CO)Ô) and a precursor of the metal M.

[0098] In a particular embodiment, as illustrated in example 1.2, the nanocrystals, for example of cesium-doped tungsten bronze, are obtained by synthesis, in oleic acid, from tungsten hexacarbonyl (W(CO)Ô) and alkali metal oleate M, for example cesium oleate.

[0099] Such a synthesis route is for example described in document

[0019] It more particularly involves the mixing, in oleic acid, of a W(CO)Ô powder and the metal oleate precursor, in particular cesium oleate, followed by heating at a temperature of at least 200°C, in particular 300°C, for a duration of at least 1 minute, in particular 30 minutes.

[0100] Surface functionalization of nanocrystals

[0101] As mentioned previously, the doped tungsten bronze nanocrystals are advantageously surface functionalized to promote their dispersion in an individualized manner within the soil formulation used to form the solar control coating according to the invention, and within said coating formed according to the invention.

[0102] According to a particular embodiment, the doped tungsten bronze nanocrystals are surface functionalized by ligands carrying hydroxyl functions.

[0103] Without wishing to be bound by theory, the interactions between the hydroxyl functions of the ligands grafted to the surface of the nanocrystal and the silanol functions of the precursors of the silica-based sol-gel matrix, as described in the rest of the text, promote the individual dispersion of each of the nanocrystals within the silica-based sol-gel network formed according to the invention. Preferably, the doped tungsten bronze nanocrystals are surface functionalized by polymer-type ligands, preferably branched polymers and more preferably hyper-branched (also called hyperbranched), carrying hydroxyl functions.

[0104] By “hyperbranched polymer” is meant branched polymer structures, comprising at least two, in particular at least three, polymer branches.

[0105] Hyperbranched polymers are generally derived from the polycondensation of one or more ABx monomers, A and B being reactive groups capable of reacting together, x being an integer greater than or equal to 2, but other preparation methods can be envisaged.

[0106] Advantageously, the doped tungsten bronze nanocrystals are surface functionalized by branched polyglycerol type ligands, preferably hyperbranched polyglycerol.

[0107] The functionalization of nanocrystals with polyglycerol can be carried out by polymerizing the polyglycerol, for example by ring-opening polymerization of glycidol, directly on the surface of the nanocrystals.

[0108] Thus, in a preferred embodiment, the doped tungsten bronze nanocrystals, preferably surface functionalized by ligands, for example ligands carrying hydroxyl functions, are prepared, prior to their use in a sol formulation according to the invention, by:

[0109] - synthesis of nanocrystals by the bottom-up route as described above, in particular in a solvent medium from tungsten hexacarbonyl (W(CO)Ô) and a precursor of the metal M and more particularly by synthesis, in oleic acid, from tungsten hexacarbonyl (W(CO)Ô) and oleate of alkali metal M, for example caesium oleate; and preferably

[0110] - functionalization of the surface of the synthesized nanocrystals by at least one ligand, in particular by at least one ligand carrying hydroxyl functions, in particular chosen from hyperbranched polymers carrying hydroxyl functions and more particularly hyperbranched polyglycerols as described previously.

[0111] Other surface functionalizations of the nanocrystals can be envisaged in order to promote the individual dispersion of the nanocrystals during the formation of the sol-gel matrix according to the invention.

[0112] As examples, nanocrystals can be surface functionalized by ligands such as functional polyphosphates or organofunctional silanes, such as gamma-glycidoxypropyltrimethoxy silane (GLYMO), (3-aminopropyl)triethoxy silane (APTES).

[0113] COATING BASED ON NANOCRYSTALS DISPERSED IN A SILICA-BASED SOE-GEE MATRIX

[0114] As previously indicated, the invention is based on the dispersion of individually doped tungsten bronze nanocrystals in a silica-based sol-gel matrix.

[0115] Dispersion of precursor(s) of the silica-based sol-gel matrix

[0116] As indicated previously, the coating is more particularly obtained from a soil formulation comprising at least:

[0117] - said doped tungsten bronze nanocrystals, in particular as defined above, preferably surface functionalized by a ligand as described above, in particular by a ligand carrying hydroxyl functions, for example hyperbranched polyglycerol, said nanocrystals being dispersed homogeneously and individually in a protic solvent medium, and

[0118] - one or more precursors of said silica-based sol-gel matrix.

[0119] When this sol formulation is deposited on a surface, the precursors condense upon evaporation of the solvent to form a network trapping the solvent. These polymerization reactions result in the formation of increasingly condensed species, which lead to colloidal particles forming gels. Drying and densification of these gels at a temperature of the order of a few hundred degrees, leads to a solid composite coating formed from a sol-gel matrix incorporating said nanocrystals. The silica-based sol-gel matrix formed according to the invention may be a silica sol-gel matrix or a mixed silica / titanium oxide or silica / zirconium oxide sol-gel matrix.

[0120] The precursors of the silica-based sol-gel matrix more particularly comprise at least organosilanes comprising hydrolyzable functions which give rise to a silica network or matrix.

[0121] Generally speaking, organosilanes can be of formula R n SiX(4- n ), in which: n is equal to 0, 1, 2, 3; the X groups, identical or different, represent hydrolyzable groups chosen from alkoxy, acyloxy or halide groups, preferably alkoxy; the R groups, identical or different, represent non-hydrolyzable organic groups linked to silicon by a carbon atom.

[0122] In particular, the sol formulation according to the invention may comprise at least, as precursor of the silica-based sol-gel matrix, in other words intended to form the sol-gel matrix, called “sol-gel precursors”, an organosilane of formula R n SiX(4- n) above-mentioned in which n is 0 or 1, such an organosilane being capable of leading to a three-dimensional network. In particular, the sol formulation according to the invention may comprise at least one organosilane precursor of which all the groups are hydrolyzable.

[0123] This sol-gel precursor is preferably a silicon alkoxide or alkoxysilane of the following formula:

[0124] [Chem 1] in which R 1 , R 2 , R 3 and R 4 , identical or different, preferably identical, preferably represent linear C 1 to C 5 alkyl chains, preferably C 1 to C 3 . In a particular embodiment, in particular when the tungsten bronze nanocrystals are surface functionalized with polyglycerol, the sol formulation uses, as sol-gel precursor, at least tetramethoxysilane or TMOS, of formula Si-(O-CH3)4. Advantageously, said sol-gel precursor(s) of alkoxysilane type, all of whose groups are hydrolyzable, is / are associated with at least one separate sol-gel precursor having at least one non-hydrolyzable group.

[0125] Said sol-gel precursor may more particularly be of the silicon alkoxide or alkoxysilane type, comprising at least one non-hydrolyzable group, preferably of the following formula:

[0126] [Chem 2] in which R, R2', R3' and R4', identical or different, preferably identical, preferably represent linear C1 to C5, preferably C1 to C3, alkyl chains, and more preferably methyl groups.

[0127] In a particular embodiment, the sol-gel precursor having a non-hydrolyzable group is methyltrimethoxysilane or MTMOS, of formula H3C-Si-(O-CH3)3.

[0128] The addition of such a sol-gel precursor, in particular MTMOS, makes it possible to influence the mechanical properties of the coating formed, by relaxing the silica sol-gel network formed. In particular, it makes it possible to avoid fracturing of the coating fdm formed after cooling to room temperature.

[0129] According to a particular embodiment, the dispersion for forming a coating according to the invention thus uses, as precursors of the silica-based sol-gel matrix, a mixture of at least one sol-gel precursor of alkoxysilane type in which all the groups are hydrolyzable and at least one sol-gel precursor of alkoxysilane type in which at least one group is not hydrolyzable.

[0130] Advantageously, the dispersion uses at least a mixture of TMOS and MTMOS.

[0131] Preferably, the sol-gel precursor comprising a non-hydrolyzable functional group, for example MTMOS, is present in the dispersion in a molar proportion less than or equal to that of the sol-gel precursor all of whose groups are hydrolyzable, for example TMOS.

[0132] Advantageously, the molar ratio between said sol-gel precursor(s) of alkoxysilane type in which all the groups are hydrolyzable, and said sol-gel precursor(s) of alkoxysilane type in which at least one group is not hydrolyzable, in particular the molar ratio TMOS:MTOS, is strictly greater than 1, in particular between 6:4 and 9:1, preferably 7.5:3.5.

[0133] In particular, said organosilane precursor(s), in particular alkoxysilane, are present in the soil formulation according to the invention in a content of between 0.12 mol / L and 9 mol / L, in particular between 1 mol / L and 5 mol / L, in the soil formulation.

[0134] Of course, the invention is not limited to the sol-gel precursors described above, and other precursors may be considered provided that they lead to the formation of a silica-based sol-gel matrix.

[0135] In particular, in the case of the formation of a mixed silica / titanium oxide or silica / zirconium oxide sol-gel matrix, the sol formulation according to the invention may comprise a mixture of one or more sol-gel precursors of organosilane type, in particular alkoxysilane, as described above, and one or more titanium or zirconium alkoxides.

[0136] Preferably, the molar quantity of said precursor(s) of titanium alkoxide type (respectively zirconium alkoxide) may be between 0 and 100% of the quantity of silane, in particular between 0 and 30%.

[0137] Also, the sol formulation according to the invention can use one or more sol-gel precursors comprising at least one non-hydrolyzable functional group capable of providing specific properties to the sol-gel matrix formed, for example a coloring, a hydrophobic, oleophobic, anti-fouling, anti-icing character, etc. In particular, the sol formulation according to the invention can use gamma-glycidoxypropyltrimethoxysilane (GLYMO) as a sol-gel precursor comprising at least one non-hydrolyzable functional group, to improve the resistance of the coatings formed, in particular with respect to cracking problems which may occur during drying or heat treatments.

[0138] Likewise, the sol formulation, and therefore the coating formed according to the invention, based on the silica-based sol-gel matrix may optionally comprise, in addition to said nanocrystals according to the invention, other particles, for example pigments and / or photocatalytic systems.

[0139] The protic solvent medium of the sol formulation used to form a solar control coating according to the invention may be formed from a single protic solvent or a mixture of practical solvents.

[0140] Said solvent(s) may be more particularly chosen from water, alcohols containing from 1 to 5 carbon atoms, such as methanol, ethanol, propan-1-ol, and mixtures thereof.

[0141] According to a particular embodiment, the sol formulation according to the invention comprises a mixture of water and one or more C1 to C5 alcohols. Preferably, the practical solvent medium is a mixture of water and methanol.

[0142] The practical solvent medium advantageously represents from 40 to 99% by volume of the sol formulation according to the invention, in particular from 60 to 99% by volume.

[0143] Nanocrystals can be implemented at a rate of 1 to 50 mg / mL in the soil formulation, in particular 1 to 15 mg / mL.

[0144] The soil formulation according to the invention can be formed by mixing the various ingredients at room temperature. Advantageously, the mixture is subjected to agitation, for example ultrasonic agitation, to allow good homogenization and dispersion.

[0145] Preferably, the soil formulation is sonicated, prior to application, for a period of 10 to 90 minutes, for example 30 minutes.

[0146] Coating formation

[0147] The invention also relates, according to another of its aspects, to the use of a sol formulation as defined above to form a coating on the surface of a support. In the context of the present invention, the term "support" refers to a solid base structure, on at least one of the faces of which a coating according to the invention is formed. The support can be of various natures, depending on the desired application.

[0148] It can be a flexible or rigid support. It can be of various shapes and geometries, depending on the application for which the coating is intended. The support can be flat or not.

[0149] Preferably, the support has good transparency properties. It advantageously has a transmittance, over the entire visible spectrum, greater than or equal to 70%, in particular greater than or equal to 80%, in particular greater than or equal to 90% and more particularly greater than or equal to 95%.

[0150] Transmittance represents the light intensity passing through the said support over the visible spectrum. It can be measured, for example, by UV-Vis spectrometry, for example using a Shimadzu UV-3100 spectrometer.

[0151] The support can thus be a support made of glass or transparent polymers such as polycarbonate, polyolefins, polyethersulfone, polysulfone, phenolic resins, epoxy resins, polyester resins, polyimide resins, polyetherester resins, polyetheramide resins, polyvinyl acetate, cellulose nitrate, cellulose acetate, polystyrene, polyurethanes, polyacrylonitrile, polytetrafluoroethylene (PTFE), polyacrylates such as polymethyl methacrylate (PMMA), polyarylate, polyetherimides, polyether ketones, polyether ether ketones, polyvinylidene fluoride, polyesters such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), polyamides, zirconia, or their derivatives.

[0152] In a particular embodiment, the support is made of glass. The coating based on the silica-based sol-gel matrix according to the invention advantageously has good affinity with the glass support.

[0153] The support may in particular have a thickness of between 500 nm and 1 cm, in particular between 10 pm and 5 mm.

[0154] For example, the support may be in the form of a window to which it is desired to give solar control properties, in particular for which it is desired to maintain good transparency in the visible range, but to screen UV and near infrared radiation. The invention also relates to a support, preferably transparent, having on at least one of its faces, a coating formed from a sol formulation according to the invention.

[0155] The method for preparing a coating according to the invention is of course adapted with regard to the configuration of the support to be coated.

[0156] Generally speaking, as mentioned above, the formation of the coating involves the steps of (ii) depositing a layer of said dispersion on the surface of the support and (iii) drying the layer to form the coating based on said silica-based sol-gel matrix.

[0157] The sol formulation may be applied to the surface of the substrate to be coated by any liquid phase deposition technique known to those skilled in the art. For example, the deposition in step (ii) of said sol formulation is carried out by spin coating, slot-dye coating, blade-coating, spraying, dip-coating, etc.

[0158] In a particular embodiment, the sol formulation is applied by spin-coating. The deposited dispersion layer may have a thickness of between 20 nm and 10 pm, in particular between 100 nm and 5 pm.

[0159] The deposited layer advantageously has a uniform thickness.

[0160] Thus, the deposited layer is preferably homogeneous, in composition and thickness.

[0161] The sol-gel coating is obtained by hardening the dispersion and thus comprises the product resulting from the hydrolysis and condensation of said precursor(s) of said silica-based sol-gel matrix, in particular said organosilane(s) as described above, optionally in a mixture with one or more titanium or zirconium alkoxides.

[0162] The drying is carried out under conditions conducive to the condensation of the sol-gel precursors and the removal of said solvent(s), to form the silica-based sol-gel network. In particular, it may be carried out at a temperature of between 40°C and 250°C, in particular approximately 100°C, in particular for a period of between 1 hour and 48 hours, in particular between 3 hours and 24 hours.

[0163] The film or coating obtained is thus based on the silica-based sol-gel matrix formed from the sol-gel precursors, in particular as defined previously, in which the doped tungsten bronze nanocrystals are dispersed, in an individual and homogeneous manner.

[0164] The coating may have a thickness of between 10 nm and 25 pm, in particular between 30 nm and 10 pm, and more particularly between 100 nm and 7 pm.

[0165] The nanocrystals may be present in the composite coating formed in a volume fraction ranging from 0.1 to 30% by volume, in particular from 0.5 to 15% by volume. Preferably, the volume fraction of nanocrystals in the composite coating is less than or equal to 5%, in particular does not exceed 3%, for example between 0.5 and 2.7%.

[0166] Advantageously, the distance between the nanocrystals within the coating formed according to the invention is strictly greater than the largest dimension of the nanocrystals. In particular, the distance between nanocrystals may be strictly greater than 4 nm and less than or equal to 100 nm, in particular between 10 nm and 50 nm. This distance may be evaluated by analyzing the images obtained by 3D tomographic transmission electron microscopy, as illustrated in the examples which follow.

[0167] It is understood that a structure according to the invention comprising at least one support having on at least one of its faces said solar control coating according to the invention may further comprise one or more additional layers, for example an anti-scratch, anti-reflection layer, a multi-layer stack of the Bragg mirror type, etc., in particular depending on the intended application.

[0168] In particular, a structure according to the invention may further comprise a protective layer on the surface of the solar control coating. In particular, the solar control coating may be interposed between the support and the protective layer.

[0169] In particular, the protective layer can be made of amorphous silicon; SitN x OyCzH uwith t being between 0 and 1, x between 0 and 4 / 3, y between 0 and 2, z between 0 and 1 and u between 0 and 4, in particular in SitNx with t between 0 and 1 and x between 0 and 4 / 3, or in S1O2, preferably in S13N4 or S1O2; AI2O3; Z1O2; ZnO; Ag; Al; in polymer, in particular chosen from a polyvinyl alcohol (PVA), a polyvinylpyrrolidone (PVP), a polymethyl methacrylate (PMMA) and a poly(butyl acrylate) (PB A); or their mixtures. Preferably, the protective layer may be made of Si3N4, SiCL, AI2O3 or their mixtures. The protective layer may also be made of a polymer chosen from PVA, PVP, PMMA and PBA. Advantageously, the protective layer can be made of a material with high transparency. For example, it can be deposited by evaporation, chemical vapor deposition (CVD), sputtering or by liquid means.

[0170] As mentioned previously, a solar control coating according to the invention can be used on various objects, for various applications.

[0171] The structure according to the invention can be more particularly implemented for glazing, for example a window, for example in buildings, verandas, windshields of automobile-type vehicles, train glazing for example, portholes, or even to equip greenhouses used in agriculture or even the surface of photovoltaic panels.

[0172] Of course, the implementation of a solar control coating according to the invention is not limited to the applications described above, and other applications of a soil formulation and / or a coating according to the invention can be envisaged.

[0173] The invention will now be described by means of the following examples and figures, given of course for illustrative purposes and not as a limitation of the invention.

[0174] Example

[0175] In the following examples, the characterizations were carried out as follows.

[0176] X-ray analysis (XRD)

[0177] X-ray diffraction (XRD) patterns were obtained using a Brucker D8 Advance diffractometer with a Cu Ka X-ray operating at 40 kV and 40 mA. Data are collected between 29 = 5° and 29 = 90° with a step size of 0.02° and a scanning speed of 0.9 sec / step. For analysis, nanocrystals were deposited by a drop-casting technique on a (400) oriented silicon wafer substrate.

[0178] Fourier transform infrared spectroscopy (FTIR)

[0179] Fourier transform infrared spectroscopy (FTIR) measurements were performed using nanocrystal pellets (2 wt%) in KBr on a Bruker Equinox 55 spectrometer in transmission mode.

[0180] Dynamic Light Scattering (DLS)

[0181] Measurements were performed on a Zeta-Sizer Nano ZS device with a total of three measurements per sample.

[0182] Scanning electron microscopy (SEM)

[0183] A Hitachi 4800 SEM scanning electron microscope (SEM) was used for imaging the films on a silicon wafer (surface and cross-section).

[0184] Extinction spectra of fdms

[0185] The extinction spectra of the films were measured using a LabSpec 4 ASD spectrophotometer connected to the glove box with optical fibers.

[0186] Reflectivity measurements

[0187] Reflectivity measurements were performed on a UMA device of an Agilent-Cary5000 spectrophotometer, with an incidence angle of 5° and a detector placed at 10°.

[0188] Image processing

[0189] The resulting reconstructions were processed using ImageJ software. First, a hybrid 3D median filter was applied to reduce noise, followed by a binarization of the volume with a common threshold calculated using a maximum entropy method. Another 3D media filtering step was used on the binary volume with a structuring object size of 2x2x2 nm 3. The volume was binarized with a threshold calculated from the image using an Otsu method and the segmentation / labeling was done using the “3D object counter” plugin, which also allows extracting the coordinates of the centroids of the nanocrystals and their volume. The Voronoi algorithm was calculated using the centroid position with the “Tess library on Pithon”. It creates 3D polyhedra (called Voronoi cell), each containing a particle. The faces of these polyhedra are defined as the assembly of points at tangential distances between two neighboring particles.

[0190] Example 1

[0191] 1.1 Synthesis of nanocrystals of different form factors

[0192] The Schlenk line technique was used for the synthesis of these nanocrystals. Oleic acid (technical grade, 90% from Sigma Aldrich) was used as a solvent and degassed for 3 hours at 120°C before synthesis.

[0193] To obtain form factors of 0.5 and 0.8, 652 mg (2 mmol) of WCL was mixed with 10 mL of oleic acid (0.2 M) and degassed for 30 minutes at 120°C to prepare the tungsten oleate precursor. In separate vials, 56 mg (0.33 mmol) and 1060 mg (6 mmol) of CsCl (Sigma Aldrich) were mixed with 45 mL of oleic acid for form factors of 0.5 and 0.8, respectively. The mixture was degassed for 30 minutes at 120°C, then heated to 300°C under nitrogen, and then 5 mL (1 mmol) of the tungsten oleate precursor was injected rapidly.

[0194] To obtain form factors of 1.8 and 2.9, the cesium oleate precursor was prepared by degassing a 0.2 M solution of CS2CO3 (Sigma Aldrich) in oleic acid for 30 minutes at 120°C. Then, 156 mg (0.44 mmol) of W(CO)Ô (Sigma Aldrich) was mixed with 73 pL (0.015 mmol) and 36.5 pL (0.007 mmol) of the cesium oleate precursor and 20 mL of oleic acid for form factors of 1.8 and 2.9, respectively. The mixture was degassed for 30 minutes and then heated to 300°C.

[0195] To obtain a form factor of 6.2, the tungsten oleate precursor was prepared by leaving 10 mL of 0.1 M W(CO)O solution in oleic acid at 180 °C in an oil bath under nitrogen flow overnight. 1 mL of this solution was injected into a 0.75 mM cesium oleate precursor solution in oleic acid at 300 °C.

[0196] For all form factors, the reaction mixtures were maintained at 300°C for 30 minutes and then cooled to room temperature. The reaction flask was transferred to a glove box under nitrogen atmosphere, the nanocrystals were washed by centrifugation, and then dispersed in toluene at a concentration of 10 mg / mL. The nanocrystals obtained were analyzed by transmission electron microscopy (TEM) and their dimensions, measured on more than 200 particles, are reported in the following Table 1.

[0197] Their morphology is also represented in figure 1a.

[0198] [Table 1]

[0199] The extinction spectra of the different nanocrystals are measured in solution in TCE (tetrachloroethylene) and presented in Figure 1a. It is observed that the form factor of the nanocrystals allows to adjust the position and spectral shape of the localized surface plasmon resonance (LSPR) peak.

[0200] 1.2 Synthesis of CsxWCh-y nanocrystals (also denoted “h-Cs AVO3”) (hexagonal structure)

[0201] Cs-type nanocrystals x WO3- y , nanorod type, with hexagonal crystallographic structure, noted h-Cs:WÛ3, presenting a form factor, noted AR (for "Aspect Ratio" in Anglo-Saxon terminology), of 1.9 (width of 6.8 ± 1.5 nm and length of 12.7 ± 4.5 nm) were synthesized according to the following protocol using a process of heating a W(CO)Ô powder and a cesium oleate precursor, obtained from cesium carbonate (CS2CO3) in oleic acid.

[0202] The Schlenk line technique (vacuum gas collector) was used for the synthesis of these nanocrystals.

[0203] Oleic acid (technical grade, 90% from Sigma Aldrich) was used as a solvent and degassed for 3 hours at 120°C before synthesis. 236 mg (1 mmol) of CS2CO3 was mixed with 10 mL of oleic acid (0.2 M cesium) and degassed for 30 minutes at 120°C under vacuum to prepare the cesium oleate precursor.

[0204] Then, 156 mg (0.44 mmol) of W(CO)O was mixed with 326 μL (0.06 mmol) of cesium oleate and 39.67 mL of oleic acid. The mixture was degassed for 30 minutes under vacuum, then heated to 300°C under nitrogen atmosphere, held for 30 minutes and cooled to room temperature.

[0205] The reaction flask was transferred to a glove box, washed with 2-propanol, and the nanocrystals were dispersed in toluene.

[0206] Figure 1b presents a transmission electron microscopy (TEM) photograph of the obtained nanocrystals. High-resolution transmission electron microscopy (HRTEM) analysis confirms that the obtained nanocrystals are indeed monocrystalline while X-ray analysis (XRD) images (Figure 2) confirm the hexagonal phase of tungsten bronze.

[0207] Example 2 (comparative)

[0208] Preparation of a film based on nanocrystals functionalized on the surface by DMOAP ligands

[0209] 2.1. Preparation of nanocrystals functionalized on the surface by DMOAP ligands (denoted

[0210] After their synthesis described in Example 1.2, the nanocrystals, visually examined, stored in toluene tend to aggregate and flocculate over time. To prevent this phenomenon, the oleic acid ligands surrounding the nanocrystals are exchanged with DMOAP ligands (N,N-dimethyl-N-[3-(trimethoxysilyl)propyl]-l-octadecanaminium chloride), according to the following protocol.

[0211] 10 mg of nanocrystals (2.5 mg / mL in toluene) were mixed with 100 pL of DMOAP (42% in methanol) and placed under sonication for one hour, then washed with ethanol.

[0212] The exchange of oleic acid ligands with DMOAP-type ligands increases their colloidal stability in a non-polar solvent, such as toluene or tetrachloroethylene.

[0213] Figure 3 shows the extinction spectrum of nanocrystals coated with oleic acid (in an aggregation state) and with DMOAP (dispersed state) at the same concentration (0.0667 mg / mL): the aggregated particles have a higher extinction coefficient in the visible due to the scattering of the aggregates while their LSPR is less intense due to the coupling effect between the nanocrystals

[0011] .

[0214] It appears from this test that the dispersion state of plasmonic nanocrystals has a major impact on their optical properties.

[0215] 2.2. Preparation of a film based on NC@DMOAP nanocrystals

[0216] A film based on NC@DMOAP nanocrystals was prepared by spin-coating at 1000 rpm - 90 sec, of a 60 mg / mL solution of NC@DMOAP nanocrystals in toluene. A compact and homogeneous film of NC@DMOAP is obtained.

[0217] Figure 4 represents the scanning electron microscopy (SEM) image of the cross section of the resulting film.

[0218] Figure 5 shows the spectrum of the extinction coefficient s of the nanocrystals dispersed in solution and in the film, calculated according to the Beer-Lamber Law: [Math. 1]

[0219] A = s . l. fV with A representing the absorption at a specific wavelength (absorption maximum);

[0220] I the optical path length through the sample (corresponding to the film thickness); and fv the volume fraction of nanocrystals (set at 70% for the

[0012] ,

[0020] film).

[0221] This representation is normalized by the total volume of nanocrystals and by the geometry of the sample and allows easier comparison between different configurations.

[0222] It can be observed that the LSPR characteristic for the film is degraded compared to the solution, as expected due to the LSPR coupling effect when the nanocrystals are stacked; the transparency in the visible is also degraded.

[0223] The selectivity towards near-infrared radiation (NIR) for solar protection applications can be evaluated by convolving their spectra with solar radiation (Figure 5), and by calculating their solar energy transmission selectivity, called "SETS" (for "Solar energy transmittance selectivity" in English terminology)

[0018] , as well as the percentage of NIR absorption, noted ANIR, for a transmittance towards the visible fixed at 80%. The values ​​of SETS and ANIR, calculated in accordance with the SI, are gathered in the following Table 2.

[0224] [Table 2]

[0225] The transition from solution-dispersed nanocrystals to compact films leads to a loss of 36% of ANIR and 58% of the extinction coefficient.

[0226] Also, the obtained values ​​of solar energy transmission selectivity and NIR absorption percentage, SETS = 0.778 and ANIR = 75.6%, for a well-dispersed solution testify to a better selectivity obtained with Cs nanocrystals. x WO3- y with a precise morphology (shape factor of 1.9), compared to Cso,33W03 powders (SETS = 0.713 and ANIR = 72.6%)

[0018] , These results highlight the advantage of precisely controlling the size and shape of the particles, and therefore the position of their LSPR, using bottom-up synthesis.

[0227] The loss of transparency in the visible wavelengths comes from the reflectivity of the film (Figure 6) resulting from the refractive index of the nanocrystals. The film is also reflective in the NIR, which accounts for 25% of the total extinction in this wavelength range. The effect of the nanocrystals being packed as a dense layer on absorption can be estimated by subtracting the reflectivity from the extinction.

[0228] It appears that the LSPR coupling between the nanocrystals at the level of the formed dense film is responsible for a 72% decrease in absorption in the NIR.

[0229] Example 3

[0230] Preparation of a composite film according to the invention based on nanocrystals functionalized on the surface by polyglycerol

[0231] 3.1. Preparation of nanocrystals surface-functionalized with hyperbranched polyglycerol (denoted “NC @ hyperbranched polyglycerol”) The nanocrystals synthesized in oleic acid as described in example 1.2 were surface-functionalized with polyglycerol-type ligands, following the protocol described below, adapted from the literature

[0022] ,

[0023] .

[0232] 1 mL of a solution of nanocrystals in toluene, as synthesized in Example 1.2, was added to 1.25 mL of glycidol in a glass tube sealed with Teflon. The tube was placed in a microwave oven; the temperature was increased to 120°C and maintained for 2 hours. After the reaction, the mixture was cooled to room temperature, washed with acetone and by repeated ultrafiltration (10,000 daltons), in methanol to remove free polymers.

[0233] Microwave heating initiates the polymerization reaction by ring opening of glycidol on the surface of the nanocrystals; the hyperbranched polyglycerol polymers obtained have numerous hydroxyl groups (Figure 7) ensuring good solubility in water and methanol by establishing hydrogen bonds.

[0234] Results

[0235] The surface-functionalized nanocrystals with polyglycerol have the same shape and dimensions (7.2 ± 1.7 nm x 13.7 ± 4.9 nm) before and after functionalization, which shows that the reaction does not attack the surface of the nanocrystals and that the ligand shell surrounds an individual nanocrystal.

[0236] Fourier transform infrared spectroscopy (FTIR) (Figure 8) confirms the efficiency of the grafting of polyglycerol ligands: the absorption band at 1700 cm' 1(C=O valence vibration) attributable to oleic acid is no longer visible after grafting. On the other hand, the absorption bands at 1096 cm' 1 (COC valence vibration), 2900 cm' 1 (valence vibration C-H2) and at 3380 cm' 1 (valence vibration -OH of hydrogen bonds), are the signature of polyglycerol.

[0237] Absorption bands in the region of 500-1030 cm' 1 correspond to the W-0 units of h-CsWOa particles.

[0238] Thermogravimetric analysis (Figure 9) indicates that the glycerol shell represents 56% of the total mass of the sample. The ligand density reported at the surface of the nanocrystals can be estimated at approximately 40 monomers / nm 2 , which corresponds to a lower density of polyglycerol molecular chains and varies depending on the length of the polymer chain.

[0239] The good dispersion of the functionalized nanocrystals in methanol was verified by the visual appearance of the solution which does not scatter light (Figure 10) and by TEM imaging showing nanocrystals well dispersed on the grid (Figure 11).

[0240] Furthermore, dynamic light scattering (DLS) measurements confirm a monodisperse particle distribution with a hydrodynamic radius of 20.8 ± 0.7 nm, which is consistent with the dimensions of the nanorods and the radius obtained for DMOAP-functionalized nanocrystals (NC@DMOAP) in toluene (18.1 ± 3.1 nm).

[0241] 3.2. Formation of the composite film according to the invention of functionalized nanocrystals in a sol-gel matrix

[0242] A mixture of tetramethoxysilane (TMOS) and methyltrimethoxy silane (MTMOS) is chosen as precursors for the formation of the sol-gel composite. Hydrolysis of TMOS and MTMOS produces methanol.

[0243] The silica sol-gel matrix incorporating the hyperbranched NC@polyglycerol nanocrystals is formed as follows.

[0244] 0.75 molar equivalents of TMOS were mixed with 0.35 molar of MTMOS and 4 molar equivalents of FhO at pH 1. The solution was mixed for one hour, diluted to the desired concentration in methanol and then added to the hyperbranched NC@polyglycerol nanocrystals.

[0245] For all samples, the nanocrystal concentration was set at 9 mg / mL and the amount of silica was increased. The solutions were sonicated for 30 minutes to initiate condensation, then deposited by spin-coating at 1000 rpm - 90 sec. The films were then placed on a hot plate at 100°C overnight.

[0246] Each film was formed, on the one hand, on a silicon wafer for electron microscopy analysis and, on the other hand, on glass for optical measurements. Without wishing to be bound by theory, the individual dispersion of each of the nanocrystals in the network is favored due to the strong interactions between the numerous -OH groups of the ligands grafted to the surface of the nanocrystal and the silanol functions of the silica precursors.

[0247] The addition of MTMOS improves the mechanical properties of the layer by relaxing the silica network. The molar ratio of TMOS:MTMOS, which advantageously prevents fracturing of the layer after cooling to room temperature, is advantageously 7.5:3.5.

[0248] The FTIR spectrum of the composite film (Figure 8) shows a significant absorption band at 1063 cm' 1 (Si-O-Si asymmetric valence vibration) associated with weak bands at 960 cm' 1(Si-OH valence vibration) and 3470 cm' 1 (valence vibration -OH), which confirms the high rate of condensation

[0024] .

[0249] Example 4

[0250] Characterization of silica sol-gel matrix film incorporating hyperbranched NC@polyglycerol nanocrystals

[0251] Several composite coatings with different volume fractions of nanocrystals (vf from 0.9% to 14.6%) in the silica matrix were prepared, as described in Example 3, to study the impact of their structure on their optical properties.

[0252] The precise structures were studied by electron tomography. Films with nanocrystal volume fractions fv = 1.1% and fv = 2.7% were reduced by focused ion beam (FIB) to a thickness of 200 nm.

[0253] 2D projections by TEM analysis at an angle of 0° (Figure 12) always show good dispersion of the nanocrystals in all cases.

[0254] The fast Fourier transform method highlights the existence of a border contrast which corresponds to the families of crystal planes of the space group P63 / mcm with a hexagonal crystal lattice, associated with the h-Cs nanocrystal x WO3- y .

[0255] Figure 13 shows the segmented 3D model for each volume fraction extracted from the tomography data. The elongation of the nanocrystals in the z direction can be observed, originating from the "missing corner" in the acquisition of the

[0025] tilt series: it induces a loss of resolution in the direction parallel to the beam direction, thus deforming the nanocrystal. Their volume distribution was extracted from the analyzed volume and compared to their distribution by standard TEM of nanocrystals deposited by drop deposition technique on carbon grids. The obtained difference of 33% can be attributed to this deformation, confirming the reliability of our image processing and highlighting the individual dispersion of the nanocrystals.

[0256] The homogeneity of the nanocrystal dispersion was analyzed using the Voronoi cell algorithm and compared to the random arrangement of particles with the same particle density. This calculation is often used to characterize granular dispersions, as it allows describing the local environment of each particle. The nanocrystals were defined by the positions of their center of gravity rather than their surface area to overcome the elongation artifact in the z direction.

[0257] Three parameters were extracted from the Voronoi tessellation, summarized in the following Table 3.

[0258] The number of nearest neighbors of each nanocrystal, denoted Nneighbors (corresponding to the face numbers of the cells in the Voronoi tessellation) presents in all cases a monodisperse distribution (Figure 14), which means that each nanocrystal has the same local environment. Moreover, the distributions are centered around 14-15 neighbors which corresponds to a random stacking of individual spheres in the literature. Furthermore, the histograms of the local volume fraction f v, local (defined as the average volume of nanocrystals divided by the cell volume) are also monodisperse with a mean value corresponding to the macroscopic volume fraction and a standard deviation corresponding to that of the nanocrystal size. These two elements and the good agreement between experiments and simulations suggest a homogeneous dispersion for both samples. Finally, the histograms of distance between nearest neighbors, denoted dN-N have the same shape between experiment and simulation (Figure 15). The slight difference in mean value can be attributed to the electrostatic repulsion between nanocrystals in the experiment. [Table 3]

[0259] Parameters extracted from Voronoi tessellation for samples with nanocrystal volume fractions of 1.1% and 2.7% for experiments and simulations.

[0260] In conclusion, the coatings formed according to the invention do indeed exhibit an individual and homogeneous dispersion of nanocrystals in the TMOS:MTMOS sol-gel matrices at least up to a volume fraction of nanocrystals of fv=2.7% (corresponding to 200 mg / mL).

[0261] The main difference in the structure of the different films for fv < 2.7% is thus the distance between the nanocrystals.

[0262] Example 5

[0263] Effect of film nanocrystal volume fraction on optical properties

[0264] The normalized extinction spectra of composite films with different nanocrystal volume fractions, prepared as described in Example 3, are reported in Figure 16. It can be seen that the transparency in the visible region increases as fv decreases. This effect is attributed to the decrease in reflectivity of the films with increasing silica content. Furthermore, the more diluted the nanocrystals are, the narrower and more blue-shifted the NIR extinction becomes.

[0265] The effect of film structure on the LSPR of nanocrystals was analyzed by examining only the absorption (taken as extinction - reflectivity). The peak position is red-shifted as fv increases due to the combined effect of LSPR coupling and the change in the refractive index of the surrounding medium of the nanocrystal.

[0266] In addition to the redshift of the LSPR position, LSPR coupling also impacts the nanocrystal extinction coefficient. For a nanocrystal volume fraction above 2.7%, this leads to a deviation from the linearity of the Beer-Lambert law (Figure 17) as a degradation of the extinction coefficient. Here, the slope of the reference line is taken as the extinction coefficient of the nanocrystal in a well-dispersed solution. This deviation begins at a center-to-center distance between nanocrystals of 20.0 ± 4.6 nm, which corresponds to a surface-to-surface distance of 10.0 ± 2.3 nm corresponding to the average nanocrystal size. This is consistent with the plasmon hybridization model established for metal nanoparticles, indicating that LSPR coupling between two particles begins to occur when the surface-to-surface distance between them is approximately equal to the particle diameter.

[0267] Thus, the optimal nanocrystal content in the film to maintain good selectivity in the NIR and a good extinction coefficient is fv < 2.7%. In fact, the coating with fv = 1.1% has optical parameters of NIR absorbance ANIR = 74.2%, solar energy transmittance selectivity (SETS) = 0.770 and extinction coefficient θ = 15.3 pm' 1 (figure 18).

[0268] These results are very close to the parameters in well-dispersed solution and superior to those obtained for milled powders dispersed in silica (SETS = 0.757 and ANIR = 71.4%)

[0018] . In addition, ANIR is calculated for a visible transmission of 80%, which is achieved here with a coating thickness of 6.0 pm corresponding to two layers spin-coated on top of each other.

[0269] Example 6

[0270] Effect of a protective layer on the stability of the extinction spectrum

[0271] Nanocrystals with a form factor of 0.5 were synthesized as described in Example 1.1.

[0272] The nanocrystals were surface functionalized with hyperbranched polyglycerol according to a protocol similar to that detailed in example 3 and dispersed in methanol.

[0273] Gamma-glycidoxypropyltrimethoxysilane (GLYMO) is chosen as the precursor for the formation of the sol-gel composite. The composite incorporating the functionalized nanocrystals of form factor 0.5 is formed as follows:

[0274] 0.97 mL of water pH=1.0 (HCl) is added dropwise into 4 mL of GLYMO and the mixture is left stirring for hydrolysis for 2 hours. 0.172 g of aluminum acetylacetonate (Al(acac)3) is then added and the solution is stirred until completely dissolved. The solution of nanocrystals functionalized with polyglycerol, dispersed in methanol, with a concentration of 10 mg / mL is added with an amount allowing to obtain in-fine in the matrix a volume fraction of nanocrystals around f v = 1.1%. After sonication for 30 min, the solution is deposited by spin coating at 1000 rpm for 90 s on a glass substrate previously cleaned by pyranha treatment. The layers obtained are dried at 100°C for 3 h to form the solar control coating.

[0275] A 50 nm thick amorphous silicon protective layer was then deposited on the solar control coating by plasma enhanced chemical vapor deposition (PECVD). The resulting structure was then left in ambient air and extinction spectra of the coating were measured over time. They are shown in Figure 19 after subtracting the contribution of amorphous silicon. No decrease in absorption was observed over a period of 26 hours, which shows that the protective layer is effective in preventing oxidation of the nanocrystals, particularly during exposure to air.

[0276] List of cited documents

[0277] [1] Schaefer et al., Surf. Coatings Technol. 93, 37-45 (1997);

[0278] [2] Martin-Palma et al., Sol. Energy Mater. Ground. Cells 55-66 (1998) doi:10.1002 / col.22225;

[0279] [3] Boisselier et al., Chem. Soc. Freeze. 38, 1759-1782 (2009);

[0280] [4] Takahata et al., J. Am. Chem. Soc. 136, 8489-8491 (2014) ;

[0281] [5] Nutz et al. J. Chem. Phys. 110, 12142-12150 (1999) ;

[0282] [6] Luther et al., Nat. Matter. 10, 361-366 (2011);

[0283] [7] Kanehara et al. J. Am. Chem. Soc. 131, 17736-17737 (2009);

[0284] [8] Dorfs, D. et al., J. Am. Chem. Soc. 133, 11175-11180 (2011);

[0285] [9] Lounis et al., J. Phys. Chem. Lett. 5, 1564-1574 (2014);

[0286]

[0010] Ohodnicki, PR et al., Thin Solid Films 539, 327-336 (2013);

[0287]

[0011] Guo, W. et al., Adv. Matter. 29, 1-9 (2017) ;

[0288]

[0012] Kim, J. et al., Nano Lett. 15, 5574-5579 (2015) ;

[0289]

[0013] Llordés et al., Nature 500, 323-326 (2013) ;

[0290]

[0014] Mattox, T. M. et al., Chem. Mater. 27, 6620-6624 (2015);

[0015] Takeda et al., J. Am. Ceram. Soc. 90, 4059-4061 (2007);

[0291]

[0016] Adachi, K. & Asahi, J. Mater. Res. 27, 965-970 (2012) ;

[0292]

[0017] Mattox et al., Chem. Mater. 26, 1779-1784 (2014) ;

[0293]

[0018] Zeng, X. et al., J. Mater. Chem. C 3, 8050-8060 (2015) ;

[0019] Kim, J., et al. Nano Lett. 16, 3879-3884 (2016) ;

[0294]

[0020] Heo, S., et al. ACS Energy Lett. 5, 2662-2670 (2020) ;

[0295]

[0021] Weisbecker, C. S., et al., Langmuir 12, 3763-3772 (1996) ;

[0296]

[0022] Zhao, L. et al.. Adv. Funct. Mater. 22, 5107-5117 (2012);

[0297]

[0023] Yu, B. et al. Colloids Surfaces A Physicochem. Eng. Asp. 596, 124734 (2020);

[0024] Capozzi, C. A. et al. Spectrosc. Lett. 26, 1335-1348 (1993) ;

[0298]

[0025] Midgley, P. A. & Weyland, M. Ultramicroscopy 96, 413-431 (2003);

[0299]

[0026] Dalmas, F. et al. 3D Dispersion of Spherical Silica Nanoparticles in Polymer Nanocomposites : A Quantitative Study by Electron Tomography. (2014) ;

[0300]

[0027] Yang, R. Y., et al., Phys. Rev. E - Stat. Physics, Plasmas, Fluids, Relat. Interdiscip. Top. 65, 8 (2002) ;

[0301]

[0028] Su, K. H. et al. Nano Lett. 3, 1087-1090 (2003).

[0302]

[0029] Hussain, A., et al., Journal of Alloys and Compounds, 246(1-2), 51-61 (1997).

Claims

Claims 1. A sol formulation, useful for forming a solar control coating, in particular a coating blocking ultraviolet (UV) and near infrared (NIR) radiation, comprising at least: - one or more silica-based sol-gel matrix precursors, and - M-type nanocrystals x W03- y , with M representing potassium (K), sodium (Na) or cesium (Cs), x ranging from 0.05 to 0.33 and y ranging from 0 to 0.4, called doped tungsten bronze nanocrystals, dispersed in a homogeneous and individualized manner in a protic solvent medium.

2. Soil formulation according to claim 1, characterized in that said doped tungsten bronze nanocrystals are surface functionalized by at least one ligand capable of promoting good dispersion of said nanocrystals within the soil formulation, said ligand(s) being in particular chosen from ligands carrying hydroxyl functions, for example polyglycerol ligands, polyphosphate ligands or organo-functional silanes, such as gamma-glycidoxypropyltrimethoxysilane (GLYMO) and (3-aminopropyl)triethoxysilane (APTES); in particular said ligand(s) being chosen from hyper-branched polyglycerols.

3. Soil formulation according to any one of the preceding claims, characterized in that said nanocrystals are cesium-doped tungsten bronze nanocrystals.

4. Soil formulation according to any one of the preceding claims, characterized in that said nanocrystals are previously obtained by an ascending synthesis route.

5. Soil formulation according to any one of the preceding claims, characterized in that said nanocrystals have a morphology of hexagonal prisms.

6. Soil formulation according to any one of the preceding claims, characterized in that said nanocrystals are in the form of nano-rods.

7. Sol formulation according to any one of the preceding claims, characterized in that said doped tungsten bronze nanocrystals have a controlled size and morphology so as to adjust the spectral position of their localized surface plasmon resonance (LSPR) peak, in particular said nanocrystals present in the form of nano-rods with a form factor between 0.1 and 20, in particular between 0.4 and 12.

8. Sol formulation according to any one of the preceding claims, characterized in that said doped tungsten bronze nanocrystals have an alkali metal doping rate, in particular caesium, of between 0.05 and 0.33 and / or a free carrier density of between 1.10 18 and 9.10 22 cm' 3 .

9. Soil formulation according to any one of the preceding claims, characterized in that said doped tungsten bronze nanocrystals are present in an amount of 1 to 50 mg / mL in the soil formulation, in particular 1 to 15 mg / mL.

10. Sol formulation according to any one of the preceding claims, characterized in that it comprises, as precursor of the silica-based sol-gel matrix, at least one organosilane of formula RnSlX(4-n), in which: n is equal to 0 or 1; the X groups, identical or different, represent hydrolyzable groups chosen from alkoxy, acyloxy or halide groups, preferably alkoxy; the R groups, identical or different, represent non-hydrolyzable organic groups linked to silicon by a carbon atom.

11. Sol formulation according to any one of the preceding claims, characterized in that it comprises, as precursors of the silica-based sol-gel matrix, at least one mixture of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMOS), in particular in a TMOS:MTOS molar ratio strictly greater than 1, in particular between 6:4 and 9:1, preferably 7.5:3.

5.

12. Soil formulation according to any one of the preceding claims, characterized in that the protic solvent medium is formed from one or more solvents chosen from water and alcohols containing from 1 to 5 carbon atoms, such as methanol, ethanol, propan-1-ol, preferably is a mixture of water and methanol.

13. Use of a soil formulation as defined according to any one of claims 1 to 12, for forming a solar control coating, in particular blocking UV and NIR radiation, on the surface of a support, in particular on the surface of a transparent support and more particularly of a support made of glass or transparent polymer(s).

14. Method for forming a solar control coating, in particular blocking UV and NIR radiation, on the surface of a support, in particular on the surface of a support made of glass or transparent polymer(s), comprising at least the steps consisting of: (i) having a sol formulation as defined according to any one of claims 1 to 12, comprising at least one or more silica-based sol-gel matrix precursors and doped tungsten bronze nanocrystals, preferably surface-functionalized by ligands as defined in claim 2, in particular ligands carrying hydroxyl functions, said nanocrystals being dispersed in a homogeneous and individualized manner in a protic solvent medium; (ii) depositing a layer of said soil formulation on the surface of said support; and (iii) drying the layer formed in step (ii) so as to obtain said silica-based sol-gel matrix.

15. Method according to the preceding claim, in which the doped tungsten bronze nanocrystals, preferably surface functionalized by ligands, are previously prepared by: - synthesis of nanocrystals by bottom-up method, in particular in a solvent medium from tungsten hexacarbonyl (W(CO)Ô) and a precursor of the metal M and more particularly by synthesis, in oleic acid, from tungsten hexacarbonyl (W(CO)Ô) and oleate of alkali metal M, for example caesium oleate; and preferably - functionalization of the surface of the synthesized nanocrystals by at least one ligand as defined in claim 2, in particular a ligand carrying hydroxyl functions, in particular chosen from hyperbranched polymers carrying hydroxyl functions and more particularly hyperbranched polyglycerols.

16. A method according to claim 14 or 15, wherein the deposition in step (ii) of said sol formulation is carried out by spin coating, slot dye coating, dip induction, blade coating or spraying; preferably by spin coating.

17. Process according to any one of claims 14 to 16, in which the drying in step (iii) is carried out at a temperature between 40°C and 250°C, in in particular of approximately 100°C, in particular for a period of between 1 hour and 48 hours, in particular between 3 hours and 24 hours.

18. Structure comprising at least one support, preferably transparent, in particular made of glass or transparent polymer(s), having on at least one of its faces a solar control coating, in particular blocking UV and NIR radiation, formed from a soil formulation as defined according to any one of claims 1 to 12, in particular obtained according to the method of any one of claims 14 to 17.

19. Structure according to the preceding claim, in which said solar control coating has a thickness of between 10 nm and 25 pm, in particular between 30 nm and 10 pm, and more particularly between 100 nm and 7 pm.

20. Structure according to claim 18 or 19, in which said solar control coating has a volume fraction of doped tungsten bronze nanocrystals of between 0.1 and 30%, in particular between 0.5 and 15%, more particularly less than or equal to 5%, preferably not exceeding 3%.

21. Structure according to any one of claims 18 to 20, in which the distance between the doped tungsten bronze nanocrystals within said coating is strictly greater than 4 nm and less than or equal to 100 nm, in particular between 10 nm and 50 nm.

22. Structure according to any one of claims 18 to 21, the coating having a transmittance, over the entire visible spectrum, greater than or equal to 70%, in particular greater than or equal to 80%.

23. Structure according to any one of claims 18 to 22, the coating having a percentage of NIR absorption, noted ANIR, greater than or equal to 60%, in particular greater than or equal to 70%, and / or a selectivity of transmission of solar energy, called “SETS”, greater than or equal to 0.70, in particular greater than or equal to 0.

75.

24. Structure according to any one of claims 18 to 23, further comprising a protective layer on the surface of the solar control coating, said protective layer being in particular made of amorphous silicon; SitN x OyCzH u with t being between 0 and 1, x between 0 and 4 / 3, y between 0 and 2, z between 0 and 1 and u between 0 and 4, in particular in SitNx with t between 0 and 1 and x between 0 and 4 / 3, or in SiCE, preferably in SiaN4 or SiCE; AI2O3; Z1O2; ZnO; Ag; Al; in polymer, in particular chosen from a polyvinyl alcohol (P VA), a polyvinylpyrrolidone (P VP), a polymethyl methacrylate (PMMA) and a poly(butyl acrylate) (PBA); or their mixtures, preferably ShN4, SiOr, AI2O3 or their mixtures.

25. Article comprising at least one structure as defined according to any one of claims 18 to 24, said article being in particular glazing, for example for windows of a building, verandas, portholes, windshields of automobile-type vehicles, train glazing, greenhouses used in agriculture or even photovoltaic panels.

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