EP4688662A1

EP4688662A1 - Production of thermochromic vanadium dioxide particulate material

Info

Publication number: EP4688662A1
Application number: EP24715916.3A
Authority: EP
Inventors: Kimberly Francine Danny TIMMERS; Luc LEUFKENS; Janique Francis Bertina HUPPERETZ; Daniel Enrico MANN; Pascal Jozef Paul Buskens
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO; Chemelot Scientific Participations BV
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO; Chemelot Scientific Participations BV
Priority date: 2023-03-27
Filing date: 2024-03-27
Publication date: 2026-02-11
Also published as: WO2024205406A1; NL2034435B1

Abstract

Embodiments pertain to doped and undoped thermochromic vanadium dioxide (VO2) particulate material, and to a method of preparing thermochromic vanadium dioxide (VO2) particulate material. The method involves providing a monoclinic VO2(M) precursor and a capping agent.

Description

P133322PC00 TITLE: PRODUCTION OF THERMOCHROMIC VANADIUM DIOXIDE PARTICULATE MATERIAL Field Aspects of the invention pertain to the production of thermochromic materials. A particular aspect of the invention is to provide thermochromic coatings, films, and materials having very advantageous low thermochromic switching temperature, advantageous low level of dopant, and good thermochromic properties. Embodiments provide methods of preparing thermochromic vanadium dioxide (VO2) particulate material. Also provided are methods for incorporating thermochromic material in a polymeric film and a method of applying the film comprising thermochromic material on an insulating glass unit. Introduction Crystalline vanadium (IV) oxide, VO2, exhibits switchable thermochromic properties thanks to a fully reversible first-order metal-insulator transition between a low temperature monoclinic phase VO2(M) and a high temperature tetragonal rutile phase VO2(R). The tetragonal rutile phase is a conductor, reflecting and absorbing a wide range of solar wavelengths in the infrared. The monoclinic phase is a semiconductor transmitting solar and infrared light wavelengths. Therefore, VO2 is a promising candidate for use in window coatings to obtain insulating glass units for buildings which block more (near) infrared from sunlight with increasing temperatures. This can advantageously be used to significantly decrease energy consumption of building sector, especially in countries with a continental or oceanic climate. In those countries, up to 8.8% of energy consumption can be reduced in the heating, ventilation, and air conditioning systems, in comparison to HR++ windows. The thermochromic switching temperature of VO2 (M) is 68°C, while an optimal thermochromic switching temperature for glass windows is 15-30°C. Doping with metals can be used to decrease the switching temperature e.g. to -10-50°C. However, typical metals used for doping have relatively high costs and it is desired to achieve a suitably low switching temperature with a lower content of doping metals or even without doping metals. Moreover, the addition of dopants may lead to compromised thermochromic properties. WO 2022/010354 describes methods for the preparation of crystalline monoclinic VO2 (M). Calvi et al., Solar Energy Materials & Solar Cells, 2022, 111783, describes the size reduction of monoclinic VO2 (M) by bead milling. The size of VO2 particulate after bead milling is reduced from approx.1 µm to approx.130-250 nm, as determined by Scanning Electron Microscopy (SEM) analyses. Bead milling of VO2 (M) powders is a top-down approach. A loss of crystallinity of 20-40% during milling was observed. Additionally, the bead milled VO2 particulate characteristics are benchmarked against hydrothermally synthesised VO2 particulate with a size of approx. 60 nm (as determined by SEM analyses). Additionally, monoclinic VO2 (M) was obtained in the hydrothermal synthesis from vanadium (V) oxide, V2O5, with hydrazine as a reducing agent. Calvi et al., Solar Energy Materials & Solar Cells, 2021, 110977 describes sol-gel synthesis of VO2 particulate doped with W. Doping beneficially reduces the structural phase transition temperature. There remains a desire for a more straightforward method of obtaining preferably highly crystalline VO2 particulate with reduced size below 100 nm to exclude light scattering. Such material may provide for reduced haze level of a film or a coating comprising the particles. Such particulate should exhibit a structural phase transition temperature, preferably in an optimal range of 15-30°C for thermochromic glazing. Insulating glass units comprising VO2 particulate coating are desired to have excellent optical properties, such as high visible light transmittance. Furthermore, it is desired to reduce the amount of dopant. Summary The invention pertains in a first aspect to an undoped thermochromic vanadium dioxide (VO2) particulate material exhibiting a phase transition temperature Tswitch for the switch from a monoclinic to a tetragonal rutile phase between 0°C and 40°C, as determined by Differential Scanning Calorimetry (DSC). The invention also pertains to a W-doped thermochromic VO2 particulate material exhibiting a phase transition temperature T switch for the switch from a monoclinic to a tetragonal rutile phase, as determined by Differential Scanning Calorimetry (DSC), which is, as a function of W-dopant amount, Tswitch ≤ a*x+b- 20ºC, where: a = -22.15 °C/atm.% W, x is W content in atm.% on the basis of total V and W atoms, and b = 68.19 °C. The invention also pertains to a W-doped thermochromic VO2 particulate material exhibiting a phase transition temperature T switch for the switch from a monoclinic to a tetragonal rutile phase, as determined by Differential Scanning Calorimetry (DSC), which is, as a function of W-dopant amount, Tswitch ≤ a*x+b- 20ºC, where: a = -22.15 °C/atm.% W, x is W content in atm.% on the basis of total V and W atoms, and b = 68.19 °C. The invention also pertains to a W-doped thermochromic VO2 particulate material, wherein said material comprises dopant (W) in an amount in the range between 0.8 atm.% and 1.5 atm. % of W, on the basis of total V and W atoms, wherein the phase transition temperature T switch for the switch from monoclinic to tetragonal rutile phase is between 15°C and 30°C, as determined by Differential Scanning Calorimetry (DSC). The invention also pertains to a method of preparing thermochromic vanadium dioxide (VO2) particulate material, the method comprising: a) providing a monoclinic VO2(M) precursor and a capping agent, b) dispersing the components specified in step a) in a liquid medium, preferably comprising water, to form a dispersion, c) subjecting the dispersion obtained in step b) to a thermal treatment to obtain the VO2 particulate material. Embodiments pertain to doped and undoped thermochromic vanadium dioxide (VO2) particulate material, and to a method of preparing thermochromic vanadium dioxide (VO2) particulate material. The method involves providing a monoclinic VO2(M) precursor and a capping agent. Brief description of the drawings Figure 1 shows DSC thermographs (from a heating cycle) of inventive samples A and B and a reference sample. Figure 2 shows DSC thermographs (from a heating cycle) of inventive sample C and a reference sample. Any embodiments illustrated in the figures are examples only and do not limit the invention. Detailed description The present invention is, in one aspect, based on the surprising result that a highly desirable thermochromic VO2 particulate product can be obtained via a top- down solvothermal or hydrothermal size reduction, as seen Example 1 (which example does not limit the inventio). This method was found to provide thermochromic VO2 particulate with unique and hitherto unattainable properties. The invention in an aspect also provides a thermochromic VO2 particulate wherein the enthalpy of thermochromic switching is high, close to the maximum achievable enthalpy, and a phase transition temperature Tswitch for the switch from monoclinic to tetragonal rutile phase (thermochromic switch temperature) is relatively low. The skilled person understands that the switch from the monoclinic phase to the tetragonal rutile is observed in a heating cycle with Differential Scanning Calorimetry (DCS). The obtained thermochromic VO2 particulate can be incorporated in a film and in an insulating glass unit without causing haze. Very surprisingly, it was found that such thermochromic VO2 particulate can be obtained via solvothermal or hydrothermal method starting from a monoclinic VO2(M) precursor, as seen in Example 1. Without wishing to be bound by way of theory, this may involve a unique top-down solvothermal, preferably hydrothermal, size reduction. State-of-the-art method of top-down size reduction is, for example, a bead milling, i.e. a solid state process. A benefit of the inventive method is reduced energy consumption compared to bead milling, which eventually would affect the production costs. Advantageously, the crystallinity of the monoclinic VO2(M) precursor is preserved, i.e. substantially all obtained VO2 particulate maintains its crystallinity and substantially no structural defects are induced during the synthesis. In particular, this advantageously allows for reducing dopant amount, such as W, for effectively reducing thermochromic switch temperature and minimalizing the decrease of thermochromic switching enthalpy. Therefore, VO2 particulate, doped and undoped, obtained via the top-down hydrothermal size reduction is of higher quality than VO2 particulate obtained via conventional top-down size reduction methods, such as bead milling. Alternatively, VO2 particulate can be obtained via bottom-up synthesis, such as hydrothermal synthesis starting from organometallic complexes. However, those methods involve the use of harmful or highly reactive substances, such as hydrazine, to ensure functional VO2 (M) particles with a size below 100 nm, which is a safety hazard. Additionally, bottom-up synthesis methods may lead to formation of mixed crystalline phases, which can be advantageously avoided in the inventive method. Without wishing to be bound by theory, it is believed that the monoclinic VO2 (M) particles are hydrothermally (or solvothermally) broken down to yield VO2 particulate with reduced size during the treatment, without dissolution and recrystallization of VO2 (M) particles and/or VO2 particulate. The present invention provides, in an aspect, an undoped thermochromic vanadium dioxide (VO2) particulate material that exhibits a phase transition temperature Tswitch for the switch from the monoclinic phase to the tetragonal rutile phase in the range from 40°C to 60°C; wherein Tswitch can be determined by Differential Scanning Calorimetry (DSC) using a heating cycle. Such thermochromic VO2 particulate advantageously exhibits promising thermochromic performance and can advantageously be used for energy-saving window glazing. In another aspect, the invention provides a thermochromic VO2 particulate material further comprising a dopant, such as W. The doped thermochromic VO2 particulate material exhibits a phase transition temperature Tswitch for the switch from monoclinic to tetragonal rutile phase, as can be determined by Differential Scanning Calorimetry (DSC) using a heating cycle. The switching temperature Tswitch,ref of a reference thermochromic VO2 (M) particulate material is defined as a function of the dopant amount x, which follows the formula Tswitch,ref = a*x+b, wherein a = -22.15 °C/atm.% dopant and b = 68.19 °C. Herein, a indicates the average Tswitch reduction per atm.% dopant, x is the dopant content in atm.% and b indicates the average Tswitch of undoped VO2 (M) particles; wherein the dopant is preferably W. Preferably, the inventive thermochromic VO2 particulate material exhibits Tswitch for the inventive thermochromic VO2 powder at least 20°C lower than Tswitch,ref calculated using the formula above. Therefore, the inventive thermochromic VO2 particulate preferably exhibits Tswitch ≤ a*x+b-20ºC, wherein a = -22.15 °C/atm.%, x is dopant content in atm.%, preferably W content, and b = 68.19 °C. Without wishing to be bound by theory, the inventors believe that the inventive VO2 particulate obtainable by the top-down solvothermal or hydrothermal size reduction method described herein, substantially maintains the crystallinity of its precursor, the precursor monoclinic VO2 (M) particles, and substantially no structural defects are induced during the treatment. Any loss of crystallinity in size-reduced VO2 particulate and the presence of structural defects is believed to negatively affect the reduction in thermochromic switch temperature. Therefore, the thermochromic VO2 particulate material in embodiments with W dopant advantageously exhibits lower phase transition temperature Tswitch for the switch from monoclinic to tetragonal rutile phase than state-of-the-art thermochromic VO2 particulate materials comprising the same amount of W-dopant. Hence, a lower amount of dopant can be used to achieve the same performance in terms of phase transition temperature Tswitch compared to conventional W-doped thermochromic VO2 particulate materials. Reduction of the amount of W dopant advantageously leads to the reduction of the production costs. Such W-doped thermochromic vanadium dioxide (VO2) particulate advantageously exhibits promising thermochromic performance and can advantageously be used for energy-saving window glazing. In another aspect, thermochromic VO2 particulate material further comprises a dopant, such as W. In particular, W-doped thermochromic VO2 particulate material comprises dopant (W) in an amount of at least 0.8 wt.% and/or less than 2.5 wt.% or less than 1.5 atm.% of W, and exhibits the phase transition temperature Tswitch for the switch from monoclinic to tetragonal rutile phase is above 15°C and below 30°C, as can be determined by Differential Scanning Calorimetry (DSC) using a heating cycle. The amount of dopant (W) is given relative total vanadium (V) and dopant atoms present in the precursor. The inventive W-doped thermochromic VO2 particulate material advantageously exhibits lower phase transition temperature Tswitch for the switch from monoclinic to tetragonal rutile phase than state-of-the-art thermochromic VO2 particulate materials comprising the same amount of W-dopant. Hence, lower amount of dopant can be used to achieve the same performance in terms of phase transition temperature Tswitch compared to conventional W-doped thermochromic VO2 particulate materials. Reduction of the amount of W dopant advantageously leads to a decrease in production costs. The inventive doped or undoped thermochromic VO2 particulate advantageously exhibits promising thermochromic performance and can advantageously be used for energy-saving window glazing. Advantageously a high enthalpy of thermochromic switching is suitably obtained for undoped and W-doped thermochromic VO2 particulate according to the invention. The enthalpy of thermochromic switching, Hswitch, of W-doped thermochromic VO2 (M) particles in general decreases as a function of thermochromic switch temperature. Furthermore, loss of crystallinity negatively affects the enthalpy of thermochromic switching. A theoretical maximum enthalpy for fully crystalline undoped and doped thermochromic VO2 (M) particles is defined as a function of thermochromic switch temperature, which follows the formula Hswitch, theoretical (also referred to as Hs, t) = a*x+b, where: a = 0.45 kJ*kg^-1*°C^-1, x is Tswitch in °C, and b = 20.28 kJ*kg^-1 (standard enthalpy of VO2 particulate at Tswitch = 0°C). Preferably, the enthalpy of thermochromic switching of the inventive thermochromic particulate material is Hswitch ≥ 0.9 * Hs, t, more preferably ≥ 0.95 * Hs, t; wherein Hs, t is said theoretical maximum enthalpy for fully crystalline thermochromic VO2 (M) particles. This advantageous high enthalpy of thermochromic switching also may indicate advantageous high crystallinity. Such enthalpy is for instance obtained with W doped material with 2.0 atm% W. Preferably, the undoped and W-doped thermochromic VO2 particulate comprises at least 80%, or at least 85%, or at least 90%, or at least 95% of particles having a largest dimension below 500 nm, or below 200 nm, or below 100 nm as can be determined by Scanning Electron Microscopy (SEM), all percentages based on number of particles. Advantageously, the undoped and W-doped thermochromic VO2 particulate material with a large part of the particles having a largest dimension of less than 200 nm can provide optically transparent films and coating. Moreover, the large fraction of particles having a largest dimension below 100 nm excludes light scattering and reduces haze level of a film or a coated insulating glass unit. Preferably the W-doped thermochromic VO2 particulate comprises at least 90%, or at least 95% of particles having a largest dimension below 200 nm. The invention also pertains to a thermochromic (solid) coating, preferably as a thermochromic inorganic coating or a thermochromic inorganic-organic hybrid coating, comprising the thermochromic VO2 particulate material. The invention also pertains to a coated article comprising a substrate and the coating. The substrate is for example a glass panel or a polymer panel, preferably as described hereinafter. The invention also pertains to a thermochromic polymeric film and/or thermochromic polymeric sheet comprising the inventive thermochromic VO2 particulate material. Preferably, the thermochromic polymeric film and/or sheet comprises a polymer selected from the group consisting of polyvinyl butyral (PVB), poly (ethylene-vinyl acetate) (EVA), polyolefin elastomers (POE), polycarbonate (PC), polyethylene terephthalate (PET), and combinations thereof. The thermochromic polymeric film and/or sheet comprises the thermochromic VO2 particulate as a particulate material, and a polymer matrix. The particulate material is embedded in the polymeric matrix. The thermochromic polymeric film and/or sheet may optionally comprise additives. The film is for instance a single layer film or a multilayer film. The film comprises e.g. a layer comprising the thermochromic VO2 particulate material and a polymeric matrix and optionally comprises additional layers; these layers are e.g. laminated. The additional layers e.g. include a polymer film. The film comprising the thermochromic VO2 particulate material is e.g. adhesive or self-adhesive. The preparation of the thermochromic film is not particularly limited and any of the known conventional methods can be used. Non-limiting examples thereof include solvent casting, extrusion, compression moulding and injection moulding. The thermochromic film is e.g. laminated with other films to produce a laminated film comprising particles of the thermochromic VO2 material. The invention also pertains to such a method of preparing a thermochromic film using the thermochromic material. The invention also pertains to a thermochromic window for architectural or vehicle application comprising the thermochromic coating or the thermochromic polymer film comprising the thermochromic VO2 particulate material. Preferably, the thermochromic window comprises a glass panel or a polymer panel and the thermochromic coating or the thermochromic polymer film. The panel is typically optically transparent for visible light. Preferably, the polymer panel comprises a polymer selected from the group consisting of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polyurethane (PU), and combinations thereof. The thermochromic window is, for instance, a single-pane glazing, or a multiple-pane glazing (viz. an insulating glass unit), such as e.g. a double-pane glazing or a triple-pane glazing;. The thermochromic window optionally comprises additional layers; these layers are e.g. laminated. The additional layers e.g. include a polymer film. Preferably, the thermochromic window further comprises a frame, and optionally a spacer bar, and optionally a desiccant. The thermochromic window is e.g. prepared by a method comprising applying the thermochromic coating or the thermochromic polymer film on the glass or polymer panel. Optionally, the thermochromic window is secured within a frame. The invention also pertains to such a method of preparing a thermochromic window using the thermochromic film or coating. The methods for manufacturing the window are not particularly limited and any of the known conventional methods can be used. Non-limiting examples thereof include the preparation of the double-pane glazing or the triple-pane glazing. Such method comprises an assembly of two or more panels, a spacer, such as aluminium or polymer, and an edge sealant. Subsequently, a gas space is filled by a low thermal conductance gas, such as argon. The thermochromic window, in particular insulating glass unit (IGU), comprises, for instance, a first pane coated with a low-e coating and a second pane coated with the thermochromic coating or laminated with the thermochromic film. Preferably, all coatings will face the closed off gas space, where the thermochromic coated or laminated pane is provided on an outer panel. Preferably, in the triple- pane glazing, two panes are coated with a low-coating. In some embodiments, the thermochromic VO2 particulate material is provided as a powder, or as part of a liquid coating composition comprising a liquid carrier and a binder, such as a Si-based binder, for instance an alkoxysilane. Preferably, the VO2 particulate material is obtainable by the hydrothermal treatment of a VO2 powder, preferably of particulate monoclinic VO2(M) precursor. Preferably, the thermochromic VO2 particulate material comprises a capping agent. The capping agent advantageously binds to a surface of the thermochromic VO2 particle to provide for colloidal stability to the particulate, especially during hydrothermal treatment, preventing particulate from agglomeration by electrostatic interactions or steric hinderance. Preferably, the capping agent is a polymer, more preferably, the capping agent is polyvinylpyrrolidone (PVP). Preferably, the molecular weight of the PVP capping agent is at least 10 kDa, or at least 20 kDa, or at least 30 kDa, or at least 40 kDa, or at least 50 kDa, or at least 60 kDa, or at least 70 kDa, or at least 80 kDa, or at least 90 kDa, or at least 100 kDa. More preferably, the molecular weight of PVP is in the range of between 10 kDa and 100 kDa, such as between 20 kDa and 60 kDa, or between 30 kDa and 50 kDa, measured with, for example, size exclusion chromatography. Without wishing to be bound by the theory, the inventors surprisingly found out that the molecular weight of PVP influences the colloidal stability of the thermochromic VO2 particulate material. The invention also pertains to a method of preparing a thermochromic vanadium dioxide (VO2) particulate material, preferably for preparing one or more of the inventive thermochromic materials. Preferably, the prepared thermochromic material has a phase transition temperature Tswitch for the switch from the monoclinic to the tetragonal rutile phase above 0°C and/or below 50°C, or below 40ºC. Preferably the thermochromic VO2 particulate material comprises a dopant in an amount up to 2 atm.%, more preferably the amount of dopant is between 0.8 atm.% and 1.5 atm.%, wherein the amount of dopant is given relative to total vanadium (V) atoms and dopant atoms present in the precursor; and the dopant is e.g. W. The inventive preparation method generally comprises: a) providing a VO2 particulate precursor, preferably a monoclinic VO2(M) precursor, and a capping agent, b) dispersing the components specified in step a) in a liquid medium (liquid component), preferably a high-boiling temperature polar solvent, e.g. water, to form a dispersion, c) subjecting the dispersion obtained in step b) to a thermal treatment, preferably solvothermal or hydrothermal treatment, to obtain the VO2 particulate material. The thermal treatment, preferably hydrothermal treatment, can be used for size reduction of the VO2 particulate precursor, preferably the monoclinic VO2(M) precursor. In the thermal treatment, preferably solvothermal treatment, the dispersion of the VO2 particulate precursor with the capping agent and the liquid medium is subjected to a treatment at elevated temperature, e.g. above 100°C, and for instance with a pressure of above 1 bar absolute, in the presence of the capping agent, and in the presence of the liquid medium components, e.g. in the presence of water. The treatment is preferably carried out in a sealed vessel, e.g. in an autoclave. The liquid component is e.g. liquid at 20ºC and 1 bar. For example, the liquid medium is a polar solvent with a high boiling temperature (≥100ºC boiling temperature at 1 bar absolute), that is for instance selected from a list consisting of water, ethylene glycol, DMAc, DMF, DMSO, n-butanol, higher alcohols, and cyclohexanol. Preferably the solvent has a relative polarity of at least 0.3; with water = 1. Particularly suitable is water as the liquid medium; in this embodiment the dispersion is an aqueous dispersion and the thermal treatment is a hydrothermal treatment. Preferably, step c) is carried out at a temperature above 100°C, preferably, up to 400°C, more preferably at a temperature above 200°C and/or up to 250°C. Preferably, step c) is carried out at a pressure in the range of 1 bar to 100 bar, more preferably at a pressure of at least 10 bar and/or up to 40 bar. Preferably, step c) is carried out for at least 1 hour, more preferably for a time in the range of 1 to 96 hours, most preferably for a time in the range of between 10 hours and 48 hours, and preferably for said duration at 100 – 400ºC or 200-250ºC, and at a pressure of at least 1 bar or at least 10 bar. Generally step c) is carried out in the presence of the liquid medium components, i.e. with the particulate matter dispersed in the liquid medium during the treatment. The method can be carried out as a batch process or as a continuous process. Advantageously, the method yields the VO2 particulate material with a reduced particle size, preferably below 200 nm, more preferably below 100 nm, as can be determined by Scanning Electron Microscopy (SEM). Advantageously, VO2 nanoparticles with a particle size of less than 200 nm result in optically transparent films and coating when incorporated. More advantageously, VO2 particulate with a particle size of less than 100 nm exclude light scattering and reduce haze level of a film or a coated insulating glass unit incorporating the particles. Preferably, monoclinic VO2(M) crystallinity of the precursor is substantially preserved and the obtained VO2 particulate material is substantially free of structural defects. More advantageously, the method yields a highly pure VO2 particulate material. Advantageously, the method does not utilize hazardous chemicals, such as hydrazine, commonly used in the bottom-up synthesis of VO2 particulate material. Moreover, the method is also low in energy demand, compared to e.g. bead milling. Preferably, the method further comprises d) reducing the temperature of the VO2 particulate dispersion obtained in step c) at least by 50°C, i.e. cooling by at least 50ºC. Preferably, the method further comprises e) isolating the VO2 particulate in solid form; preferably with cooling of the VO2 particulate dispersion and/or VO2 particulate material before, simultaneously and/or after the isolation. Optionally, the method involves washing the VO2 isolated particulate, for example with an organic solvent. The particulate is isolated by e.g. solvent evaporation, precipitation, centrifugation, filtration, spray drying or freeze drying. Isolation advantageously allows for using said particulate as e.g. as a pigment composition, and is convenient for storage, shipment, and handling of said particulate. The method comprises the use of the capping agent. The capping agent is added to advantageously prevent from the formation of mixed phases hydrates in VO2 particulate. Moreover, the capping agent advantageously may bind to the surface of the particles to provide for colloidal stability of the particulate, thereby advantageously preventing the particulate from overgrowth and agglomeration, for instance by electrostatic interactions and/or steric hinderance. The capping agent is, for example, a water-soluble polymeric molecule. Preferably, the capping agent is a polymer. More preferably, the capping agent is selected, for example, from the group consisting of polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polyethylene glycol thiol (PEG-Thiol), polyvinyl alcohol (PVA), cetrimonium bromide (N,N,N-Trimethylhexadecan-1-aminium bromide) (CTAB), sodium dodecyl sulfate (SDS), oleic acid, citric acid, maleic acid, tartaric acid, oleylamine, 1-dodecanethiol. For example, the capping agent is PVP. For instance, the amount of capping agent is in the range from 2 wt.% to 8 wt.% relative to the mass of the monoclinic VO2(M) precursor. In some embodiments, the method comprises providing a dopant in step a) of the method. The dopant is for example selected from the group consisting of W, Mo, Mn, Nb, Co, Fe, Cu, Ni, Bi, Ag, Au, Ru, Rh, Pd, Pt, Ta, Zr, Ti, Cr, Sn, Zn, In, Eu, Sm, Yb, Tb, La, Ce, Lu, Gd, Nd, Er, Dy, Pr, Tm, Ho, Ce, Be, Mg, Ca, Sr, Y, Li, Al, N, S, Cl, I, P, C, Se, Te, Be, B, F, H, and combinations thereof. Advantageously, the use of dopant may improve thermochromic and optical performance, (photo)catalytic activity and/or electrical properties of VO2 particulate. Preferably, the dopant is preferably selected from the group consisting of W, Mo, Nb, Co, Fe, Ta, Zr, Ti, Cr, Sn, Eu, Tb, La, Ce, Be, Mg, Ca, Sr, Al, Be, B, F, H and combinations thereof. Advantageously, the dopant is W. Advantageously, this allows for tuning thermochromic properties, such as the thermochromic switch temperature of the VO2 particulate material. A reduction of the amount of dopent is advantageous to lower costs, e.g. for W. For example, the dopant is an individual component or for example the monoclinic VO2(M) precursor particulate already comprises the dopant. Preferably, the monoclinic VO2(M) precursor comprises between 0 atm.% and 2 atm.% of dopants, more preferably between 0.8 atm.% and 1.5 atm.% of dopant. Advantageously, the amount of dopant is decreased compared to commonly applied amounts to reach the same decrease in Tswitch. Advantageously, this allows for obtaining doped VO2 particulate improved thermochromic performance while less dopant can be added. Furthermore, a reduction in the amount of dopant, such as W, is particularly advantageous due to its cost. The method involves providing a VO2 particulate precursor, preferably a monoclinic VO2(M) precursor, more preferably a crystalline monoclinic VO2(M) precursor. The precursor contains VO2 particles, more preferably monoclinic VO2 particles. Preparation methods for monoclinic VO2 particles are known in the art, see e.g. WO 2022/010354. For the precursor, preferably at least 80 number % of particles have a largest dimension above 1.0 µm or above 2.0 µm, or above 10 µm, as determined by Scanning Electron Microscopy (SEM). Suitably, at least 50 number % of particles have largest dimension in the range 5 – 20 µm in the precursor material. In a preferred example embodiment, the method pertains to the hydrothermal size reduction of VO2 particulate precursor. VO2 particulate obtainable via the hydrothermal size reduction using the inventive method advantageously exhibits high crystallinity and improved thermochromic properties as shown in Examples 1 and 2. Preferably, highly crystalline monoclinic VO2 (M) particulate is used as the precursor. In some embodiments, the method preferably further involves incorporating the VO2 particulate material in a film, preferably a polymeric film, or in a liquid coating composition. The film comprises the VO2 particulate material and a matrix, preferably a polymer matrix. The thermochromic polymeric film and/or thermochromic sheet may optionally comprise additives. The film is, for instance, a single layer film or a multilayer film. The film comprises e.g. a layer comprising VO2 particulate and a matrix and optionally additional layers, these layers are e.g. laminated. The additional layers e.g. include a polymer film. The film comprising VO2 particulate is e.g. adhesive or self-adhesive. A method the thermochromic film preparation is not specifically limited and any of the known conventional methods can be used. Non-limiting examples thereof include solvent casting, extrusion, compression moulding and injection moulding. The film is e.g. laminated with other films to produce a laminated film comprising VO2 particulate. The liquid coating composition comprises for instance a liquid carrier and a binder, such as a Si-based binder. In some embodiments, the method preferably further comprises step h) of applying the film or liquid coating composition specified in step g) on a pane. The pane with the applied film comprising VO2 particulate is for instance used for the production of thermochromic windows, as discussed hereinabove. The pane is for instance a glass panel or a polymer panel as described in connection with the thermochromic window. The pane for instance incorporated in an insulating glass unit. An onset switching temperature is defined as a temperature at which 2% of VO2 has transitioned from its monoclinic to tetragonal rutile phase and can be derived with DSC. In particular, the switching onset temperature corresponds to the temperature associated with a value of 2% of the integral of a heat flow change (peak). Said peak originates from the VO2 phase transition. A switching temperature, Tswitch, is defined as a tempature at which the maximum of the heat flow peak originating from the VO2 phase transition from its monoclinic to tetragonal rutile phase occurs. The switching temperature can be derived with DSC. A switching enthalpy is defined as an enthalpy corresponding to VO2 phase transition from its monoclinic to tetragonal rutile phase. The switching enthalpy can be derived with DSC. A particulate size distribution (PSD) can be derived with Scanning Electron Microscope (SEM). From the recorded SEM images, the dimensions of a sample number of particles, such as 100 particles, are determined using suitable software. From these size measurements, a number percentage of the particles having a length (largest dimension) lower than 500 nm, lower than 200 nm, and lower than 100 nm is calculated. An amount (atm.%) of dopant (e.g. W) is calculated on the basis of metal atoms (total V and dopant metal atoms, e.g. total V and W atoms). Particulate material, as used herein, indicates material comprising or consisting of (solid) particles, e.g. comprising or consisting of nanoparticles. Pressures are absolute pressures. Examples The invention will now be further illustrated by the following non-limiting examples. These examples do not limit the invention and do not limit the claims. Characterization methods The onset switching temperature, the switching temperature, and the switching enthalpy were measured with differential scanning calorimetry (DSC) using a DSC Discovery Classic system from TA Instruments®. The sample was measured in a hermetic aluminium pan under N2 atmosphere, with a heating and cooling rate of 20°C/min. The reported onset switching temperature, switching temperature, and switching enthalpy data are derived from a DSC heating cycle. In order to determine the particulate size distribution, Scanning Electron Microscope (SEM) images were recorded. From the SEM images, the size of 100 particles was measured using imageJ software. From these 100 size measurements, the percentage (by number) of the particles having a largest dimension (length or diameter) lower than 500 nm was calculated. Example 1 In this example, undoped thermochromic VO2 (M) particulate was prepared using monoclinic VO2(M) as a precursor, using a top-down hydrothermal size reduction procedure in an aqueous dispersion at a temperature in the range of 200- 245°C, at a pressure in the range 1 - 100 bar, preferably at a pressure of approx. 30 bar, in the presence of an capping agent, such as PVP, in an amount of less than 8%. Materials The precursor monoclinic VO2(M) was prepared according to a suitable method, with mostly particles having a size in the range 10-15 µm. Ethanol, PVP K30 and PVP K15 were purchased from Sigma Aldrich®. PVP K30 and PVP K15 have molecular weight of 40 kDa and 10 kDa, respectively. Milli-Q® water purified via the Q-POD-Merck™ system. Preparation of undoped thermochromic VO2 particulate In this example, particulate 0.2 g monoclinic VO2(M) was dispersed in 20 ml water in a Parr® autoclave. Subsequently, 6 wt.% of PVP K30 or 6 wt.% of PVP K15 was added. Thereafter, the autoclave was heated up to 245°C for 48 hours for PVP K30 and for 10 hours for PVP K15. After the autoclave cooled down to room temperature, the solids were isolated by centrifuging at 10000 rpm, washed with ethanol, and vacuum-dried (50°C, 50 mbar, overnight). The sample prepared with PVP K30 and 48h reaction time is denoted “A”, the sample with PVP K15 and 10 hours reaction time is denoted “B”. The precursor monoclinic VO2(M) sample is denoted as “P1”. Results Figure 1 shows DSC thermographs (from a heating cycle) of samples A, B, and P1. Table 1 provides a summary of the DSC results and the particulate size distribution (PSD) of Example 1, with PSD expressed as number % of the particles with a length (largest dimension) below 500 nm, below 200 nm, or below 100 nm, as determined by SEM analysis. Table 1 Sample Onset Switching Switching PSD PSD PSD switching temp. enthalpy (500 (200 (100 temp. nm) nm) nm) (°C) (°C) (J/g) P1 68 (±5) 71 54.0 0% 0% 0% (reference) A 5.3 (±5) 47 40.6 83% 82% 15% B 35(±5) 57 34.9 58% 5% 0% Advantageously, a lower onset switching temperature is obtained for samples A and B compared to precursor P1, especially for sample A the onset switching temperature is drastically reduced. Thereby samples A and B demonstrate examples of an undoped thermochromic VO2 particulate material exhibiting a phase transition temperature Tswitch for the switch from a monoclinic to a tetragonal rutile phase between 40°C and 60°C. Additionally, the switching temperature and the switching enthalpy are somewhat decreased by the treatment. While the decrease of the switching temperature is desirable, the decrease of the switching enthalpy should be as low as possible given its correlation with the switching temperature. Thereby sample A demonstrates an example of an undoped thermochromic VO2 particulate material exhibiting the enthalpy of thermochromic switching Hswitch of the undoped VO2 particulate with Hswitch ≥ 0.9 * Hs, t, wherein Hs, t =a*x+b, wherein a = 0.45 kJ*kg^-1*°C^-1 and b = 20.28 kJ*kg^-1 and x is Tswitch in °C. In particular, for the Tswitch of 47°C the Hs, t calculated using the above-mentioned formula is 41.4 kJ/kg, thus Hswitch ≥ 0.9 * Hs, t is above 37. kJ/kg while sample A exhibits Hswitch of 40.6 kJ/kg. Furthermore, the particle size is smaller after the treatment as shown by the PSD values. Example 2 In this example, precursor W-doped monoclinic VO2(M) was prepared according to a suitable method, which is substantially the same method as used for the precursor synthesis in Example 1. The only difference in the method of precursor synthesis is that the starting material comprises 2 atm.% of W. Precursor W-doped monoclinic VO2(M) sample is denoted as “P2”. W-doped thermochromic VO2 particulate was prepared from the doped precursor using the method according to Example 1, using PVP K30 (6 wt. %) as capping agent and 48h reaction time. The obtained W-doped thermochromic VO2 particulate sample is denoted as sample “C”. Results Figure 2 shows DSC thermographs (from a heating cycle) of the corresponding W-doped VO2 samples. Table 2 provides a summary of the DSC results and the particulate size distribution (PSD) of Example 2, with PSD expressed as number % of the particles with a length (largest dimension) below 500 nm, below 200 nm, or below 100 nm, as determined by SEM analysis Table 2 Sample Onset Switching Switching PSD PSD PSD switching temp. enthalpy (500 (200 (100 temp. nm) nm) nm) (°C) (°C) (J/g) P2 (ref.) 12 (±5) 22 35.1 0% 0% 0% C -15 (±5) -3.5 20.1 97% 97% 25% Results Advantageously, a lower onset switching temperature and lower switching temperature are obtained for the thermochromic W-doped VO2(M) particulate, compared to its W-doped monoclinic VO2(M) precursor. In particular, the onset switching temperature determined for sample C is drastically decreased. Thereby sample C demonstrates an example of a W-doped thermochromic VO2 particulate material exhibiting a phase transition temperature T switch for the switch from a monoclinic to a tetragonal rutile phase which is as a function of W-dopant amount, Tswitch ≤ a*x+b-20ºC, where: a = -22.15 °C/atm.% W, x is W content in atm.% on the basis of total V and W atoms, and b = 68.19 °C. In particular, for the W content of 2 atm.%, the Tswitch calculated using the above-mentioned formula is below 3.83ºC while sample C exhibits Tswitch of -3.5ºC. Moreover, the combination of W dopant and VO2(M) with reduced size led to very low onset switching temperature and switching temperature. Therefore, a thermochromic VO2 particulate material can be prepared with advantageously low W-dopant amount, and still exhibit a desired low onset switching temperature and switching temperature. Furthermore, sample C demonstrates an example of a W-doped thermochromic VO2 particulate material exhibiting the enthalpy of thermochromic switching Hswitch of the W-doped VO2 particulate with Hswitch ≥ 0.9 * Hs, t, wherein Hs, t =a*x+b, wherein a = 0.45 kJ*kg^-1*°C^-1 and b = 20.28 kJ*kg^-1 and x is Tswitch in °C. In particular, for the Tswitch of -3.5°C the Hs, t calculated using the above-mentioned formula is 18.7 kJ/kg, thus Hswitch ≥ 0.9 * Hs, t is above 16.8 kJ/kg while sample C exhibits Hswitch of 20.1 kJ/kg.

Claims

Claims 1. An undoped thermochromic vanadium dioxide (VO2) particulate material exhibiting a phase transition temperature Tswitch for the switch from a monoclinic to a tetragonal rutile phase between 40°C and 60°C, as determined by Differential Scanning Calorimetry (DSC).

2. A W-doped thermochromic VO2 particulate material exhibiting a phase transition temperature T switch for the switch from a monoclinic to a tetragonal rutile phase, as determined by Differential Scanning Calorimetry (DSC), which is, as a function of W-dopant amount, Tswitch ≤ a*x+b-20ºC, where: a = -22.15 °C/atm.% W, x is W content in atm.% on the basis of total V and W atoms, and b = 68.19 °C.

3. A W-doped thermochromic VO2 particulate material, wherein said material comprises dopant (W) in an amount in the range between 0.8 atm.% and 1.5 atm. % of W, on the basis of total V and W atoms, wherein the phase transition temperature T switch for the switch from monoclinic to tetragonal rutile phase is between 15°C and 30°C, as determined by Differential Scanning Calorimetry (DSC).

4. A material according to any of claims 1-3, wherein the enthalpy of thermochromic switching Hswitch of the undoped or W-doped thermochromic VO2 particulate meets Hswitch ≥ 0.9 * Hswitch, theoretical, wherein Hswitch, theoretical =a*x+b, wherein a = 0.45 kJ*kg^-1*°C^-1 and b = 20.28 kJ*kg^-1 and x is Tswitch in °C.

5. A material according to any of claims 1-4, wherein at least 80 number % of particles have a largest dimension (length) below 200 nm, as determined by Scanning Electron Microscopy (SEM).

6. A material according to claim 5, wherein at least 90 number % of particles have a largest dimension (length) below 200 nm, as determined by Scanning Electron Microscopy (SEM).

7. A solid thermochromic coating comprising the thermochromic VO2 particulate material according to any of claims 1-6; preferably wherein said coating is an inorganic coating or an inorganic-organic hybrid coating.

8. A thermochromic polymer film and/or sheet comprising the thermochromic VO2 particulate material according to any of claims 1-6; preferably wherein said polymer film and/or sheet comprises a polymer selected from the group consisting of PVB, EVA, POE, PC, PET, and combinations thereof.

9. A thermochromic window for architectural or vehicle application, wherein said thermochromic window comprises the thermochromic coating according to claim 7 or the thermochromic polymer film according to claim 8.

10. The thermochromic VO2 particulate material according to any of claims 1-6 in the form of a powder.

11. A material according to any of claims 1-10, wherein thermochromic VO2 particulate material comprises a capping agent.

12. A material according to claim 11, wherein the capping agent is a polymer.

13. A material according claim 12, wherein the capping agent is PVP.

14. A material according claim 13, wherein the molecular weight of the PVP capping agent is at least 10 kDa.

15. A method of preparing thermochromic vanadium dioxide (VO2) particulate material, the method comprising: a) providing a monoclinic VO2(M) precursor and a capping agent, b) dispersing the components specified in step a) in a liquid medium, preferably comprising water, to form a dispersion, c) subjecting the dispersion obtained in step b) to a thermal treatment to obtain the VO2 particulate material.

16. The method according to claim 15, further comprising: d) optionally reducing the temperature of the VO2 particulate dispersion obtained in step c) at least by 50°C, e) isolating the VO2 particulate in solid form from the dispersion, and optionally f) washing the isolated VO2 particulate material.

17. The method according to any of claims 15-16, wherein the capping agent is a polymer.

18. The method according to claim 17, wherein the capping agent is selected from the group consisting of PVP, PEG, and PVA.

19. The method according to claim 18, wherein the capping agent is PVP.

20. The method according to any of claims 15-19, wherein the amount of capping agent is in the range from 2 wt.% to 8 wt.% relative to the mass of the monoclinic VO2(M) precursor.

21. The method according to any of claims 15-20, wherein a dopant is provided in step a), wherein said dopant is selected from the group consisting of W, Mo, Mn, Nb, Co, Fe, Cu, Ni, Bi, Ag, Au, Ru, Rh, Pd, Pt, Ta, Zr, Ti, Cr, Sn, Zn, In, Eu, Sm, Yb, Tb, La, Ce, Lu, Gd, Nd, Er, Dy, Pr, Tm, Ho, Ce, Be, Mg, Ca, Sr, Y, Li, Al, N, S, Cl, I, P, C, Se, Te, Be, B, F, H, and combinations thereof, preferably wherein the dopant is an individual component or the monoclinic VO2(M) precursor particulate comprises the dopant.

22. The method according to claim 21, wherein the monoclinic VO2(M) precursor comprises the dopant in an amount ranging between 0 atm.% and 2 atm.% relative to total V and dopant atoms present in the monoclinic VO2(M) precursor.

23. The method according to any of claims 21-22, wherein the dopant is W.

24. The method according to any of the claims 15-23, wherein step c) is carried out at the temperature between 100°C and 400°C.

25. The method according to any of the claims 15-24, wherein step c) is carried out at the pressure in the range from 10 bar to 40 bar.

26. The method according to any of the claims 15-25, wherein step c) is carried out for a period of from 1 hour to 96 hours at out at the temperature between 100°C and 400°C and at the pressure in the range from 10 bar to 40 bar.

27. The method according to claim 26, wherein the liquid medium comprises water and step c) involves hydrothermal treatment at a temperature in the range of 200°C to 250°C.

28. The method according to any of the claims 15-27, further comprising: g) incorporating the thermochromic VO2 particulate material obtained in any of the steps c) to f), in a film or liquid coating composition.

29. The method according to claim 28, further comprising: h) applying the film or coating composition specified in step g) on a glass or polymeric panel.

30. The method according to any of claims 15-29, wherein the method is suitable for preparing a material according to any of claims 1-6 or 11-14.