Synthesis of thermochromic nanoparticles on nanoparticulate scaffolding

Abstract

A thermochromic material comprising vanadium dioxide (VO2) nanoparticles dispersed on surfaces of larger scaffolding particles such as amorphous silicon dioxide (SiO2). The SiO2 particles prevent the vanadium dioxide nanoparticles, which are typically less than approximately 40 nm in size, from sintering into larger particles, thus giving the material advantageous properties. A base added to the vanadium precursor solution functionalizes the scaffolding particles. The size of the vanadium dioxide nanoparticles can be changed by changing the size of the scaffolding particles. The material can be dispersed in a coating, film, ink, plastic, or polymer.

Description

[0001] SYNTHESIS OF THERMOCHROMIC NANOPARTICLES ON NANOPARTICULATE

[0002] SCAFFOLDING

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority to and the benefit of the filing of U.S. Provisional Patent Application No. 63 / 574,761, entitled “SYNTHESIS OF THERMOCHROMIC NANOPARTICLES ON NANOPARTICULATE SCAFFOLDING”, filed on April 4, 2024, the entirety of which is incorporated herein by reference.

[0005] BACKGROUND OF THE INVENTION

[0006] Field of the Invention (Technical Field)

[0007] The present invention is related to synthetic pathways of materials whose absorbance, particularly of infrared (IR) wavelengths, is reversibly thermally switchable, and whose optical properties preferably rely on disperse, sub-40 nm particles. Applications include coatings for windows and building exterior and roofs or other surfaces to passively modulate thermal heat gain properties, particularly from solar radiation

[0008] Background Art

[0009] Note that the following discussion may refer to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.

[0010] Heat gain and heat loss through windows are responsible for 25%-30% of residential heating and cooling energy use. Heating and cooling costs also represent the highest operating expense for commercial buildings at roughly $2 per square foot (per year) and account for 17% of CO2 emissions worldwide Additionally, in residential buildings, Americans spend 6 5% of household expenditures on energy. Manufacturers of architectural glass, window films, agricultural films, and other products are looking for solutions to help their customers reduce these costs and the negative effects of CO2 on the environment. More importantly, energy consumption is predicted to increase in the next two decades.

[0011] Smart windows are becoming increasingly important as they significantly reduce energy consumption by dynamically tuning the transmitted solar light depending via environmental triggers. In the recent years vanadium dioxide (VO2) has gained enormous interest due to its ability to thermally switch from the monoclinic (semi-conducting) phase to tetragonal (metallic) phase. This phase change enables IR rejection properties of these materials. The low temperature monoclinic (M1) phase is transparent to IR radiation, allowing it into the building, whereas the rutile (R) phase exhibits plasmon resonance with near-infrared (NIR) radiation and absorbs IR radiation based on environmental temperature. Pure VO2 has a transition temperature of 68°C, which is too high to be effective, especially for passive heat modulation. However, VC ’s thermochromic switching can be leveraged by adding dopants that shift the transition temperature to lower or higher temperatures, rendering it customizable for a specific location and application.

[0012] W6+-doped VO2 nanopowders have been synthesized by thermolysis of a [NH4]5[(VO)e(CO3)4(OH)9]-10H2O precursor at low temperature. A decrease in Tc from 67.15°C to 26.46°C in the presence of doping was demonstrated, which is thus effective in reducing the transition temperature. The band gap of a vanadium dioxide film has been shown to be control lably adjustable using a hydrothermal method for doping vanadium oxide with different metallic dopants, thereby achieving different transition temperatures.

[0013] Various nanoparticle synthesis methods include hydrothermal, solvothermal, sol-gel, precipitation, etc. Solution based methods are widely used for their lower cost, ease of doping, processibility, and scalability. Another typical approach for effective preparation of VO2 nanostructures is the oxidation or reduction of V2O3 or V2O5 by various oxidizing or reducing species in hydrothermal / solvothermal conditions with subsequent thermal treatment.

[0014] In a hydrothermal reaction, V2O5 was used as a precursor, hydrothermal reduction and precipitation occurred between 220°C and 260°C in 4 hours. In this hydrothermal reaction, oxalic acid is used as reducing agent. Oxalate forms an anionic complex with VO2+as [VO2(oxalate)2]3'. Oxalic acid is known to reduce V(V) to a deep blue colored vanadyl complex.

[0015] One of the main challenges of VO2 synthesis and processing is obtaining nanocrystals with narrow size distribution and preventing particle sintering during necessary annealing processes. Particles larger than 100 nm scatter light in the visible region, which results in a drop in optical transparency and generates an unacceptable level of haze to the end user of these materials. For metallic nanoparticles, such as VC>2 in the rutile phase, Localized Surface Plasmon Resonance (LSPR) can enhance the performance of the smart coatings. LSPR is dependent on the size of the nanoparticles; for particles that are much smaller than the incident light wavelength, LSPR will be in resonance with smaller wavelengths of external light and will be blue shifted compared to particles that are bigger. To take advantage of the nanoscale properties of VO2, it is important to explore methods that produce smaller particles and that reduce sintering during post processing by keeping the particles separate throughout. For example, a hexagonally patterned VO2 array showed a red shift of the LSPR in the NIR region as the diameter of the particle increased from 67 nm to 287 nm at critical temperatures of VO2. In another example, a VO2 composite film was prepared by ball milling VO2 under controlled conditions, and the particles were dispersed in a polymer and coated on a substrate A decrease in haze at 555 nm from 46 5% to 2 2% was produced by reducing the particle size from 150 nm to 40 nm, while the optical transmittance increased from 9.6% to 38.5% and solar modulation improved from 8.1% to 16.9%.

[0016] Another way to improve the optical properties is by the core-shell method, in which a thin shell of an optically transparent material such as SiC>2 is chemically formed using the Stober process on a VO2 core. Polyvinylpyrrolidone polymer can be used as a coupling agent, producing sub-nanometer particle sizes having a good size distribution in the polymer coating, resulting in a solar modulation efficiency of 13.6%. In another example, transition metal doped core VO2 particles coated with SIO2 and TIC>2 shells at improved visible light transmittance and suppressed oxidative degradation of transition metal doped VO2. In another example, the SiCL shell prevented sintering of the individual nanoparticles during high temperature annealing without changing the original size and shape of the core VO2 particles The vanadium oxide precursor powder was prepared by the precipitation process, followed by coating the core with a SiC>2 shell which is inert and prevented sintering during a high temperature thermal treatment at 600°C for 30 min.

[0017] SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

[0018] An embodiment of the present invention is a thermochromic material comprising vanadium dioxide (VO2) nanoparticles dispersed on surfaces of larger particles. The larger particles preferably comprise an oxide, a carbide, or a nitride and are preferably optically transparent or optically translucent The larger particles more preferably comprise amorphous silicon dioxide (SiO2) and are preferably between approximately 10 nm and approximately 1000 nm in diameter, more preferably between about 10 nm and approximately 150 nm in diameter. The vanadium dioxide nanoparticles are preferably less than approximately 40 nm in size, more preferably between approximately 3 nm and approximately 33 nm in size. The thermochromic material preferably does not comprise an oxide coating. The thermochromic material is preferably dispersed into a coating, film, ink, plastic, or polymer. Other embodiments of the invention are a coating, polymer, plastic, film, or ink comprising the thermochromic material.

[0019] Another embodiment of the present invention is a method of manufacturing a thermochromic material, the method comprising mixing a vanadium precursor solution comprising a base with a solution of suspended scaffolding particles to form a combined solution, the base thereby functionalizing the scaffolding particles; removing solvents from the combined solution to form a powder; and heating the powder in a reactive gas, thereby forming a final powder comprising vanadium dioxide (VO2) nanoparticles dispersed on surfaces of the scaffolding particles. The solution of suspended scaffolding particles preferably comprises a protic solvent. The vanadium precursor solution optionally comprises a dopant source, optionally comprising tungsten. The reactive gas preferably comprises hydrogen. The base preferably comprises ammonia, sodium hydroxide, potassium hydroxide, or pyridine. The scaffolding particles preferably comprise silicon dioxide (SiC>2) and are preferably between approximately 10 nm and approximately 1000 nm in diameter, more preferably between about 10 nm and approximately 150 nm in diameter. The vanadium dioxide nanoparticles are preferably less than approximately 40 nm in size, more preferably between approximately 3 nm and approximately 33 nm in size. The method preferably further comprises producing a first predetermined size of the vanadium dioxide nanoparticles by using scaffolding particles having a second predetermined size. The method preferably further comprises removing sintered vanadium dioxide particles that are not disposed on a scaffolding particle from the final powder; ball milling the final powder; and dispersing the final powder in a coating, film, ink, plastic, or polymer.

[0020] Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated into and form a part of the specification, illustrate the practice of embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating certain embodiments of the invention and are not to be construed as limiting the invention. In the drawings:

[0022] FIG 1 is a transmission electron microscopy (TEM) micrograph shows the morphology of vanadium dioxide nanoparticles grown on 150 nm SiO2 nanoparticles.

[0023] FIG 2 is an X-ray diffraction (XRD) pattern of a single phase VO2 nanoparticle composite of the present invention, indicating the purity of the monoclinic M1 phase at 20 °C (room temperature).

[0024] FIG 3 shows UV-NIR variable temperature data collected for the VO2 nanoparticle composite of the present invention shown in FIG. 1 at 15 °C and 80 °C.

[0025] FIG 4 is a TEM micrograph showing the morphology of vanadium dioxide nanoparticles grown in the same manner as those in FIG. 2 but without SiO2 nanoparticles as scaffolding.

[0026] FIG 5 shows UV-NIR variable temperature data collected for the VO2 synthesized without SiO2scaffolding shown in FIG. 4 at 15 °C and 80 °C.

[0027] DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0028] Embodiments of the present invention are related to novel methods for the production of thermochromic vanadium dioxide (VO2). In one or more embodiments of the present invention, a solgel of a vanadium organometallic precursor is preferably adhered to the surface of preferably amorphous oxide nanoparticles (preferably comprising silicon dioxide (SiO2)), which act as scaffolding for the growth of preferably discrete VO2 nanoparticles preferably less than about 40 nm in size. SiO2 enables the synthesis of small amorphous vanadium-based nanoparticles and annealing under reactive gases and reduces the occurrence of sintering during required annealing The resulting material is distinct from a core-shell material, as the nanoparticles on the surface of the vanadium dioxide are preferably discrete and thus behave as individuated vanadium dioxide nanoparticles. Unlike previous work in which the SiO2 forms a barrier surrounding each VO2 particle, in embodiments of the present invention the SiC>2 thus templates the formation of the VO2 nanoparticles, keeping them separated while forming on the larger SiO2 nanoparticle and reducing sintering and the resultant formation of large VO2 particles. Another advantage of the present invention is that the size of the SiO2nanoparticles can determine the size of the VO2nanoparticles. Particle size control of the VO2 nanoparticles can be achieved by varying the size of the SiO2nanoparticle scaffolding. Specifically, smaller particles sizes can be achieved using smaller scaffolding material, resulting in a powder with improved thermochromic properties.

[0029] The resulting material is preferably a dispersible powder having tunable nanoparticulatedependent thermochromic characteristics, such as enhanced LSPR, reduced cross-over in the visible region of the spectra, improved optical properties such as reduced haze, and mimicking the behavior of simulated ideal vanadium dioxide nanoparticle materials. The vanadium dioxide nanoparticles preferably comprise dopants selected to adjust the phase change temperature. Other support scaffolding materials, including but not limited to other metal oxides, carbides, and nitrides, may be used instead of the amorphous SiO2nanoparticles to provide the desired chemical stability or other properties.

[0030] There are many routes to form nanoparticles of vanadium oxide, including hydrothermal, solgel, molten salt, and chemical precipitation. To form a quality pigment, solution processing methods that form disperse, uniform particles are preferable. Many of the sol-gel and aqueous processing routes form nanocrystalline powders of both pure and doped VO2, and all require thermal treatment in controlled atmosphere, such as controlled oxygen partial pressure thermal annealing, to develop the desired crystallinity and phase of VO2, since vanadium oxide has a wide range of polymorphs and phases between vanadium metal and V2O5 Annealing processes commonly produce deleterious sintering of the nanoparticles. Although prevention of sintering of VO2materials during necessary annealing events is also achievable by formation of an SiO2shell around a nanoparticulate core of a VO2precursor, this route does not enable the user to utilize reactive gases in annealing processes, such as H2, O2, or others. Thus, in that case, the anneal is suitable only for crystallization purposes, or conversion of various polymorphs of VO2into the high temperature, thermochromic VO2(R) phase. In contrast, growing nanoparticulates of a vanadium organometallic precursor on SiO2scaffolding nanoparticles enables the use of reactive gases to produce reductive or oxidative events to obtain the desired oxidation state of vanadium. The ability to use reactive gases is highlighted in the examples. It is preferable for the process to form particles that are well-dispersed, optimized for size, crystallinity (e.g. single crystal or polycrystalline) and oxidation state, and with a homogenous incorporation of dopants Functionalized, oxidatively-stable nanoparticles, for example SIC»2 nanoparticles, preferably about 10 nm to about 1000 nm, and more preferably about 10 to about 150 nm, in size which are well- suspended in a preferably protic solvent such as H2O, EtOH, or iPrOH, are preferably mixed into and suspended in a vanadium organometallic precursor solution, for example a vanadium sol-gel precursor solution, together with one or more dopants, for example one or more metal salts of Na, W, Mo, or combinations thereof, preferably from about 0.1 to about 10 mol%, more preferably from about 1 to about 3 mol%. The SiC>2 particles can be fumed silica but are preferably formed by the Stober process. The vanadium sol-gel precursor can be synthesized from one equivalent of polymorphs of V2O5, inorganic vanadium-containing salts, or organometallic vanadium sources such as vanadium pentoxide or vanadium (V) oxytriisopropoxide, along with 1 to about 10, but preferably about 2 to about 5 equivalents of a reductant that can be organic in nature (such as oxalic acid, citric, acid, isopropanol, pyridine, etc.) or inorganic (such as hydrazine). A dopant source, such as ammonia tungstate or tungsten ethoxide (for tungsten as the dopant), may optionally be included in the precursor solution. The solvent is preferably removed and the resulting sol-gel nanoparticle composite is heated to, for example, 500 °C, preferably under an atmosphere of reactive gas for a length of time depending on the sample mass. Annealing after solvent removal preferably thermally converts the vanadium organometallic precursor to crystalline, inorganic, differentiated and / or individuated VO2 particles, preferably comprising a size of less than about 40 nm, and more preferably about 3 to about 33 nm, resulting in the formation of a thermochromic powder comprising VO2 nanoparticles on the surface of the larger SIC»2 nanoparticles, as shown for one example in FIGS. 1-2.

[0031] SiO2, being an amphoteric substance, can take on a positive, neutral, or negative charge depending on the pH of the solution in which it is suspended Negatively or positively surface charged SiO2preferably achieves a good particle suspension in vanadium organometallic sol-gel precursors. More specifically, a base comprising, for example, ammonia, sodium hydroxide, potassium hydroxide, or pyridine, is preferably combined with a vanadium sol-gel precursor solution. The base is preferably added in a mol ratio of about 1 to about 30 equivalents of base to moles of SiO2, more preferably in a range of about 1 to about 5 mmol, depending on the strength of the base. In this example the SiO2particles become negatively charged. The negative surface charge of the SiO2nanoparticles, which is likely provided by deprotonated hydroxyl groups, preferably enables ligation of the positively charged vanadium species to the SiO2 surface In one or more embodiments of the present invention the VO2 / SiC>2 composite is optionally further encapsulated in an SiC shell to allow for ease of surface functionalization based on the known surface chemistry of the SiC>2 surface, in addition to protecting the VO2 from environmental oxidation. However, for most embodiments, such an oxide shell is not required, because the composite is typically embedded in a polymer encapsulation.

[0032] The VO2 nanoparticles exhibit a thermochromic monoclinic to tetragonal structural transition, preferably resulting in an infrared thermochromic phase change temperature between about -10 °C and about 85 °C. The VO2 nanoparticles may optionally be removed from the nanoparticulate scaffolding via chemical etching, such as base, acid, oxidative, or reductive etching, high energy sonication, ball milling, or other chemical or mechanical methods. The VO2 nanoparticles (with or without the scaffolding) can then be finely dispersed into a desired formulation or matrix to create transparent or translucent materials which display a thermally stimulated transition from infrared transmissive to infrared absorptive. The formulation or matrix can be dispersed as a thin coating on, for example, window glass, window films, glass laminate interlayers, or on similar transparent surfaces, or alternatively embedded into a plastic, and preferably has a highly visible transmission preferably between about 60% and about 70%. The formulations may include other organic and / or inorganic compounds that help enable uniform dispersion of the particles or add coloration.

[0033] The suspended thermochromic pigments can be dispersed in, for example, films; inks; additives; sprayable or printable coatings applied to structural films such as ETFE or the like; polymers that may be extrudable, comprising a plastic such as acrylic, polycarbonate, polyvinyl chloride, polyethylene, high-density polyethylene, polyvinyl butyral, or ethylene tetrafluoroethylene; coatings that may be used on glass laminate interlayers, adhesive window films or suspended films in glass assemblies; coatings for fabric or textile products used in, for instance, performance clothing, tents, or tension structures; or adhesives that may be used to laminate films of the same or dissimilar compositions. The resulting coatings may be sprayed, dip-coated, roll coated, knife-coated, slot-die coated, curtain coated, printed, or applied via any method onto a substrate or object. The coating thickness is preferably between approximately 100 nm and 1 mm. These coatings, plastic polymers, adhesives, or pigments may be varied in their visible transmission, transparency, and opacity by altering the volume fraction in the matrix and particle size.

[0034] A thermochromic laminate interlayer may comprise a thermochromic pigment coated directly onto an interlayer sheet and laminated between glass panes Alternatively, laminated layers of various materials may incorporate thermochromic pigments. For example, a layer of thermochromic pigment can be coated onto a polyethylene terephthalate (PET) sheet, laminated between two layers of PVB sheet material, and then laminated between two layers of coated or uncoated glass. Composites can be designed for maximum energy savings, durability, or aesthetics.

[0035] Examples Example 1

[0036] A mixture of vanadium (V) oxytriisopropoxide (6.5 mL), a solution of tungsten (VI) ethoxide in pyridine (0.64 mL, 0.10 M), pyridine (95 mL), and deionized water (4.3 mL) was added to a beaker of acetone (743 mL) with stirring at room temperature. The solution was stirred for 30 minutes, during which time an orange precipitate was formed The precipitate was isolated with centrifugation (4255 ref x 30 minutes) and washed with acetone before being dried for in an 80 °C oven. The orange powder (0.450 g) was suspended in isopropanol ( 120 mL) via sonication and then added to a suspension of 150 nm SiC>2 nanoparticles (0.06 M, 120 mL) in isopropanol. The mixture was sonicated to ensure thorough mixing and then the solvent was removed under reduced pressure. The resulting residue was dried in an oven at 100 °C. The resulting powder (0.300 g) was annealed at 600 °C under an atmosphere of 3% H2 in N2 to generate V2O3. The V2O3 (0.100 g) was then oxidized to VO2 via annealing at 550 °C under an atmosphere of O2 in N2. FIG. 1 shows the morphology and the discrete VO2 particles on the SiC>2 surface, while FIG 2 demonstrates the phase purity of the material The VO2 nanoparticles had a diameter of about 18 (±3) nm and were dispersed on SiO2 nanoparticles of 150 (± 33) nm diameter, as measured from TEM images.

[0037] Coatings comprising the nanocomposite were made by first ball milling the composite in DI H2O at 1500 rpm; this step is preferably performed to break up the material so it will be well dispersed in the polymer matrix. This was followed by addition of polyvinylpyrrolidone (0.150 g), and the mixture was sonicated at room temperature for 15 minutes. The suspension was then centrifuged at 1000 ref for 5 minutes to remove any sintered VO2 particles that had not adhered to the surface of a SiO2 nanoparticle. The water was removed under reduced pressure and the particles were resuspended in DI water (0.4 mL) and Bondthane UD-620 (1.8 mL) as a polymer matrix. The composite was sonicated at room temperature for 15 minutes and centrifuged a final time at 1000 ref for 5 minutes to remove any unsuspended material The formulation was coated on glass slides utilizing hand-drawn Meyer rods. The pigment produced by these conditions was investigated via variable temperature UV-NIR, which probes the expected thermally triggered difference in UV-NIR absorption caused by the phase transition from M1 to R. Thermochromic performance of the material is described in terms of AT, which is the difference in optical transmittance between the cold and the hot states at a given wavelength, where the ATsoi measured is corrected for the standard Solar Energy Spectrum for North America at wavelengths 300 to 2500 nm. ATvis, and ATiR are calculated analogously but from wavelength ranges of 380 - 780 nm and 780 - 2500 nm, respectively. As shown in FIG. 3, this material exhibits no cross-over in the visible region, resulting in a ATvis of 1.8% Compared with the results for VO2 made via the same process but without SiCh nanoparticles described in Example 2 below, this result is attributable to the increased bandgap energy of the sub-50 nm VO2 particles in the semiconducting or“cold” state The ATI is 18 1%, and ATsoi, is 7 9%

[0038] Example 2

[0039] A synthesis process identical to that of Example 1 , but without the inclusion of SiC>2 nanoparticles, was performed. FIG. 4 is a TEM displaying the morphology of the VO2 nanoparticles grown without SiO2, with VO2 nanoparticles of a diameter of 102 (±14) nm as measured by TEM images. FIG. 5 shows the UV-NIR variable temperature data for this material, showing a much larger cross-over of ATvis of -1.7%, with a red-shifted onset at 740 nm. ATIR is 13.3% and ATsoi is 3 9%, demonstrating significantly lower performance than that of the material of Example 1

[0040] Note that in the specification and claims, “about” or “approximately” means within twenty percent (20%) of the numerical amount cited. As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a functional group” refers to one or more functional groups, and reference to “the method" includes reference to equivalent steps and methods that would be understood and appreciated by those skilled in the art, and so forth.

[0041] Although the invention has been described in detail with particular reference to the disclosed embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents. The entire disclosures of all patents and publications cited above are hereby incorporated by reference

Claims

CLAIMS1 . A thermochromic material comprising vanadium dioxide (VO2) nanoparticles dispersed on surfaces of larger particles.

2. The thermochromic material of claim 1 wherein the larger particles comprise an oxide, a carbide, or a nitride.

3. The thermochromic material of claim 2 wherein the larger particles are optically transparent or optically translucent.

4. The thermochromic material of claim 2 wherein the larger particles comprise silicon dioxide (SiO2).

5. The thermochromic material of claim 4 wherein the larger particles comprise amorphous silicon dioxide6. The thermochromic material of claim 1 wherein the larger particles are between approximately 10 nm and approximately 1000 nm in diameter.

7. The thermochromic material of claim 6 wherein the larger particles are between about 10 nm and approximately 150 nm in diameter.

8. The thermochromic material of claim 1 wherein the vanadium dioxide nanoparticles are less than approximately 40 nm in size.9 The thermochromic material of claim 8 wherein the vanadium dioxide nanoparticles are between approximately 3 nm and approximately 33 nm in size.

10. The thermochromic material of claim 1 not comprising an oxide coating.

11. The thermochromic material of claim 1 dispersed into a coating, film, ink, plastic, or polymer.

12. A coating comprising the thermochromic material of claim 1.

13. A polymer or plastic comprising the thermochromic material of claim 1.

14. A film or ink comprising the thermochromic material of claim 1.

15. A method of manufacturing a thermochromic material, the method comprising: mixing a vanadium precursor solution comprising a base with a solution of suspended scaffolding particles to form a combined solution, the base thereby functionalizing the scaffolding particles; removing solvents from the combined solution to form a powder; and heating the powder in a reactive gas, thereby forming a final powder comprising vanadium dioxide (VO2) nanoparticles dispersed on surfaces of the scaffolding particles.

16. The method of claim 15 wherein the solution of suspended scaffolding particles comprises a protic solvent17. The method of claim 15 wherein the vanadium precursor solution comprises a dopant source.

18. The method of claim 17 wherein the dopant source comprises tungsten.

19. The method of claim 15 wherein the reactive gas comprises hydrogen.

20. The method of claim 15 wherein the base comprises ammonia, sodium hydroxide, potassium hydroxide, or pyridine.

21. The method of claim 15 wherein the scaffolding particles comprise silicon dioxide(SiO2).

22. The method of claim 15 wherein the scaffolding particles are between approximately 10 nm and approximately 1000 nm in diameter.

23. The method of claim 22 wherein the scaffolding particles are between about 10 nm and approximately 150 nm in diameter.

24. The method of claim 15 wherein the vanadium dioxide nanoparticles are less than approximately 40 nm in size25. The method of claim 24 wherein the vanadium dioxide nanoparticles are between approximately 3 nm and approximately 33 nm in size.

26. The method of claim 15 further comprising producing a first predetermined size of the vanadium dioxide nanoparticles by using scaffolding particles having a second predetermined size27 The method of claim 15 further comprising : removing sintered vanadium dioxide particles that are not disposed on a scaffolding particle from the final powder; ball milling the final powder; and dispersing the final powder in a coating, film, ink, plastic, or polymer.

Images