Closed pore metal oxide particles

Abstract

Closed-cell metal oxide particles and methods of making the same are disclosed in certain embodiments. In at least one embodiment, the closed-cell metal oxide particles comprise a metal oxide matrix defining an array of closed cells. Each closed cell encloses an inaccessible void volume. An external surface of the closed-cell metal oxide particles is defined by the array of closed cells.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 055,011, filed July 22, 2020, the disclosure of which is incorporated by reference herein in its entirety. TECHNICAL FIELD

[0003] The present application relates to metal oxide particles having, for example, structural colorant properties and methods of making the same. BACKGROUND

[0004] Traditional pigments and dyes rely on chemical structure to exhibit color through light absorption and reflection. Structural colorants rely on physical structure rather than chemical structure to exhibit color through light interference effects. Structural colorants exist in nature, for example in bird feathers, butterfly wings, and certain gemstones. Structural colorants are materials that contain micro- or nanostructured surfaces that are small enough to interfere with visible light and produce color. For example, such materials often contain nanoscale pore structures that contribute to their optical characteristics. However, the medium exposed within the pores can influence these optical characteristics by changing the net refractive index or by changing the average refractive index within the pores. SUMMARY

[0005] The following Summary presents a simplified summary of various aspects of the disclosure to provide a basic understanding of such aspects. This Summary is not an extensive overview of the disclosure. Neither is it intended to identify key or critical elements of the disclosure or to delineate any scope of the particular implementations of the disclosure or of any scope of the claims. The sole purpose of the Summary is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description presented later.

[0006] In one aspect of the disclosure, a method of making a closed-cell metal oxide particle includes generating droplets from a particle dispersion, the particle dispersion including first particles including a polymeric material and second particles including a metal oxide material; drying the droplets to provide dried particles including an array of the first particles; and calcining or sintering the dried particles. In at least one embodiment, each of the first particles is coated by a layer of the second particles. In at least one embodiment, the calcining or sintering densifies the metal oxide material and removes the polymeric material to produce a closed-cell metal oxide particle, each closed-cell metal oxide particle including a metal oxide matrix defining an array of closed cells, each closed cell enclosing an inaccessible void volume. In at least one embodiment, an outer surface of the closed-cell metal oxide particle is defined by its respective array of closed cells.

[0007] In at least one embodiment, the array of closed cells is an ordered array. In at least one embodiment, the array of closed cells is an unordered array.

[0008] In at least one embodiment, the first particles comprise a net positive surface charge and wherein the second particles comprise a net negative surface charge. In at least one embodiment, the first particles comprise a net negative surface charge and wherein the second particles comprise a net positive surface charge. In at least one embodiment, the surface charge drives the formation of a layer of second particles on the first particles.

[0009] In at least one embodiment, the polymeric material comprises a polymer selected from poly(meth)acrylic acid, poly(meth)acrylate, polystyrene, polyacrylamide, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, methyl methacrylate and copolymer of [2-(methacryloyloxy)ethyl]trimethylammonium chloride, a derivative thereof, a salt thereof, a copolymer thereof, or a mixture thereof.

[0010] In at least one embodiment, the first particles have an average diameter of about 50 nm to about 500 nm.

[0011] In at least one embodiment, the metal oxide material comprises a metal oxide selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof. In at least one embodiment, the metal oxide material comprises silicon dioxide.

[0012] In at least one embodiment, the second particles have an average diameter of about 1 nm to about 120 nm.

[0013] In at least one embodiment, the closed-pore metal oxide particles have an average diameter of about 0.5 pm to about 100 pm.

[0014] In at least one embodiment, droplet generation is performed using a microfluidic method.

[0015] In at least one embodiment, droplet generation and drying is performed using a spray drying method.

[0016] In at least one embodiment, droplet generation is performed using a vibrating nozzle.

[0017] In at least one embodiment, drying the droplets comprises evaporation, microwave radiation, oven drying, vacuum drying, drying in the presence of a desiccant, or a combination thereof.

[0018] In at least one embodiment, the particle dispersion is an aqueous particle dispersion.

[0019] In at least one embodiment, the weight ratio of the first particles to the second particles is about 1 / 10 to about 10 / 1.

[0020] In at least one embodiment, the weight ratio of the first particles to the second particles is about 2 / 3, about 1 / 1, about 3 / 2, or about 3 / 1.

[0021] In at least one embodiment, the second particles have a particle size ratio to the first particles of 1 / 50 to 1 / 5.

[0022] In another aspect of the disclosure, a method of making closed-cell metal oxide particles includes: generating droplets from a particle dispersion, the particle dispersion including a polymer in a sol-gel matrix of a metal oxide material; the polymer particles including a polymeric material; drying the droplets to provide dried particles including an array of polymer particles; and calcining or sintering the dried particles to obtain closed-cell metal oxide particles. In at least one embodiment, each of the polymer particles is coated by the sol-gel matrix. In at least one embodiment, the calcining or sintering removes the polymeric material and densifies the metal oxide material to produce the closed-cell metal oxide particles, each closed-cell metal oxide particle including a metal oxide matrix defining an array of closed cells, each closed cell enclosing an inaccessible void volume. In at least one embodiment, an outer surface of the closed-cell metal oxide particles is defined by their respective array of closed cells.

[0023] In at least one embodiment, the polymer particles include a net positive surface charge and the sol-gel matrix of the metal oxide material includes a net negative charge. In at least one embodiment, the polymer particles include a net negative surface charge and the sol-gel matrix of the metal oxide material includes a net positive charge.

[0024] In another aspect of the disclosure, the closed-cell metal oxide particles are made by any of the foregoing methods or any of the methods described herein.

[0025] In another aspect of the disclosure, the closed-cell metal oxide particles include a metal oxide matrix defining an array of closed cells, each closed cell enclosing an inaccessible void volume. In at least one embodiment, an outer surface of the closed-cell metal oxide particles is defined by the array of closed cells.

[0026] In at least one embodiment, the array of closed cells is an ordered array. In at least one embodiment, the array of closed cells is an unordered array.

[0027] In at least one embodiment, the average diameter of the void volumes is about 50 nm to about 500 nm.

[0028] In at least one embodiment, the metal oxide matrix includes a metal oxide selected from the group consisting of silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof. In at least one embodiment, the metal oxide matrix includes silicon dioxide.

[0029] In at least one embodiment, the closed-cell metal oxide particles are at least partially derived from polymeric particles having an average diameter of about 50 nm to about 500 nm. In at least one embodiment, the closed-cell metal oxide particles are at least partially derived from metal oxide particles having an average diameter of about 1 nm to about 120 nm.

[0030] In at least one embodiment, the closed-cell metal oxide particles are derived from a metal oxide precursor selected from the group consisting of silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

[0031] In another aspect of the disclosure, a composition comprises a plurality of the closed-cell metal oxide particles according to any of the preceding embodiments or any of the embodiments described herein. In at least one embodiment, the closed-cell metal oxide particles have an average diameter in the range of about 0.5 pm to about 100 pm. In at least one embodiment, the composition further comprises a substrate having the closed-cell metal oxide particles disposed thereon.

[0032] In at least one embodiment, the closed-cell oxide particles of any of the embodiments described herein further comprise a light absorber. In at least one embodiment, the light absorber is present at 0.1 wt% to about 40.0 wt%. In at least one embodiment, the light absorber comprises carbon black. In at least one embodiment, the light absorber comprises one or more ionic species.

[0033] In another aspect of the disclosure, a bulk composition exhibiting whiteness, non-whiteness, or an effect in the ultraviolet spectrum, the bulk composition comprising a plurality of the closed-cell metal oxide particles according to any of the embodiments described herein.

[0034] Other aspects of the disclosure relate to compositions comprising the closed-cell metal oxide particles described herein in the form of an aqueous formulation, an oil-based formulation, an ink, a paint formulation, a food, a plastic, a cosmetic formulation, or a material for medical or security applications.

[0035] As used herein, the term “bulk sample” refers to a population of particles. For example, a bulk sample of particles is simply a bulk population of particles, e.g., > 0.1 mg, > 0.2 mg, > 0.3 mg, > 0.4 mg, > 0.5 mg, > 0.7 mg, > 1.0 g, > 2.5 mg, > 5.0 mg, > 10.0 mg, or > 25.0 mg. The bulk sample of particles can be substantially free of other components.

[0036] Also as used herein, the phrase “exhibits a color observable by the human eye” means that an ordinary person would observe a color. This can be used for any bulk sample distributed over any surface area, e.g., distributed over about 1 cm 2 , about 2 cm2 about 3 cm 2 about 4 cm 2 about 5 cm 2 or about 6 cm 2 about 7 cm 2 about 8 cm 2 about 9 cm 2 about 10 cm 2 about 11 cm 2 about 12 cm 2 about 13 cm 2 about 14 cm 2 or about 15 cm 2 Any of the above. It can also mean observable by a CIE 1931 2° standard observer and / or a CIE 1964 10° standard observer. The background for color observation can be any background, such as a white background, a black background, or a dark background anywhere between white and black.

[0037] Also as used herein, the term “of’ can mean “comprising.” For example, “of a liquid dispersion” can be interpreted as “comprising a liquid dispersion.”

[0038] Also as used herein, the terms “particle,” “microsphere,” “microparticle,” “nanosphere,” “nanoparticle,” “droplet,” and the like can refer to, for example, a plurality thereof, a collection thereof, a population thereof, a sample thereof, or a bulk sample thereof.

[0039] Also as used herein, the term “micron” or “micrometer” means 1 micrometer (pm) to less than 1000 pm, for example, when referring to a particle. The term “nanometer” or “nanometer” means 1 nanometer (nm) to less than 1000 nm, for example, when referring to a particle.

[0040] Also as used herein, the term “monodisperse” with respect to a population of particles means particles having a substantially uniform shape and a substantially uniform diameter. For example, a population of monodisperse particles can have 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% of the particles present having a diameter within ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, or ±1% of the population average diameter.

[0041] Also as used herein, the term “impenetrable to a medium” with respect to a volume means that the volume is shielded from the penetration of macromolecules (e.g., molecules having a molecular weight greater than 5000 g / mol, such as polymers and oligomers). The volume can be permeable to solvents, such as water, toluene, hexane, and ethanol.

[0042] As also used herein, the term "substantially free of" means containing, for example, < 5%, < 4%, < 3%, < 2%, < 1%, < 0.5%, < 0.4%, < 0.3%, < 0.2%, or < 0.1% by weight of the other component.

[0043] The articles "a" and "an" as used herein refer to one or to more than one (e.g., to at least one) of the grammatical object of the article. Any ranges of

[0044] As also used herein, the term "about" is used to describe and account for small fluctuations. For example, "about" can mean plus or minus 5%, plus or minus 4%, plus or minus 3%, plus or minus 2%, plus or minus 1%, plus or minus 0.5%, plus or minus 0.4%, plus or minus 0.3%, plus or minus 0.2%, plus or minus 0.1%, or plus or minus 0.05% of the value. All numerical values are modified by the term "about," whether or not explicitly indicated. The numerical values modified by the term "about" include the specific values recited and include the endpoints of the ranges. For example, "about 5.0" includes 5.0.

[0045] All parts and percentages are by weight unless otherwise indicated. Weight percent (wt%) is based on the entire composition, that is, on a dry solids basis, unless otherwise indicated, if not otherwise indicated. BRIEF DESCRIPTION OF DRAWINGS

[0046] The disclosure described herein is illustrated by way of example and not by way of limitation in the accompanying drawings.

[0047] Figure 1A Metal oxide particles having a closed pore morphology are depicted in accordance with some embodiments of the present disclosure.

[0048] Figure 1B A comparative metal oxide particle having a porous outer surface is depicted.

[0049] Figure 2 A method of making metal oxide particles having a closed pore morphology in accordance with some embodiments of the present disclosure is depicted.

[0050] Figure 3 A schematic of an exemplary spray drying system used in accordance with various embodiments of the present disclosure is shown.

[0051] Figure 4 Scanning electron microscope (SEM) images of closed pore metal oxide particles produced by microfluidic techniques in accordance with embodiments of the present disclosure are shown.

[0052] Figure 5 A photograph comparing closed pore silica particles produced in accordance with embodiments of the present disclosure to porous particles to demonstrate the prevention of oil penetration into the voids of the closed pore silica particles is shown.

[0053] Figure 6 An SEM image of closed-pore silica particles produced by a spray drying method according to embodiments of the present disclosure is shown.

[0054] Figure 7 A plot of UV-vis spectra of samples produced according to embodiments of the present disclosure showing a reflection peak at 440 nm corresponding to blue color.

[0055] Figure 8 A plot of UV-vis spectra of samples produced according to embodiments of the present disclosure showing a reflection peak at 520 nm corresponding to green color.

[0056] Figure 9 A plot of UV-vis spectra showing the relative attenuation values in the UV range of closed-pore silica particles and silica nanoparticles produced according to embodiments of the present disclosure.

[0057] Figure 10 An SEM image of closed-pore titania particles produced according to further embodiments of the present disclosure is shown.

[0058] Figure 11 An SEM image of closed-pore silica particles produced by a sol-gel method according to embodiments of the present disclosure is shown. DETAILED DESCRIPTION

[0059] Embodiments of the present disclosure relate to closed-pore metal oxide particles comprising a metal oxide matrix having an array of substantially uniform-sized pores (referred to as "void volumes" or "voids", which can comprise air) formed therein, as illustrated in the cross-sectional view in Figure 1A As illustrated, the closed-pore metal oxide particles are formed from a metal oxide matrix defining an array of "closed pores" that enclose a media-impenetrable void volume. The outer surface of the closed-pore metal oxide particles (depicted as an outer coated surface formed from a metal oxide) is defined by the closed-pore array, such that substantially no openings of similar size to the closed pores are present at the surface.

[0060] In contrast to the present embodiments, Figure 1B The porous metal oxide particles shown have pores on their outer surface and interconnected pores in the interior. When formulated into a medium, the medium penetrates into these pores, resulting in a loss of color effect in the downstream formulation due to the refractive index matching between the medium and the matrix material. This greatly limits the use of porous particles in various formulations. The closed-pore metal oxide particles of the present embodiments are impermeable to polymers and macromolecules that are often used in such formulations, and thus can prevent penetration into the pores and retain air in the pores. As a result, the closed-pore metal oxide particles advantageously maintain a constant net refractive index between the matrix and the void, regardless of the surrounding medium in the application.

[0061] Figure 2 An exemplary method of forming closed-cell metal oxide particles is illustrated. In certain embodiments, the closed-cell metal oxide particles are produced by drying droplets of a formulation that includes metal oxide particle substrates of 1 to 120 nm in diameter and polymer particles of 50 to 500 nm in diameter that will serve as templates. In certain embodiments, the two types of particles are oppositely charged (e.g., positively charged polymer particles and negatively charged metal oxide particles) to facilitate the formation of a coating of metal oxide particles on the polymer particles. In certain embodiments, the droplets (e.g., aqueous droplets) are generated using a spray drying or microfluidic process, and the droplets are dried to remove their solvent. In certain embodiments that utilize a spray drying process, the generation and drying of the droplets are performed rapidly and continuously. During the drying process, the polymer particles and the metal oxide particles self-assemble to form microspheres containing polymer particles embedded in a metal oxide matrix. The matrix nanoparticles are densified and form a stable matrix around the polymer particles, for example, by sintering the matrix nanoparticles in a muffle furnace. During this process, the polymer particles are removed by calcination, resulting in the final closed-cell particles with a closed-cell array formed therein.

[0062] The resulting closed-cell metal oxide particles can be micrometer-sized, for example, with an average diameter of about 0.5 pm to about 100 pm. In certain embodiments, the closed-cell metal oxide particles have an average diameter of about 0.5 pm, about 0.6 pm, about 0.7 pm, about 0.8 pm, about 0.9 pm, about 1.0 pm, about 5.0 pm, about 10 pm, about 20 pm, about 30 pm, about 40 pm, about 50 pm, about 60 pm, about 70 pm, about 80 pm, about 90 pm, about 100 pm, or within any range defined by any of these average diameters (e.g., about 1.0 pm to about 20 pm, about 5.0 pm to about 50 pm, etc.). The metal oxide employed can also be in the form of particles, and can be nanometer-sized. The average diameter of the metal oxide matrix particles can be, for example, about 1 nm to about 120 nm. The average diameter of the polymer template particles can be, for example, about 50 nm to about 500 nm. One or more of the polymer particles or the metal oxide particles can be polydisperse or monodisperse. In certain embodiments, the metal oxide can be provided as metal oxide particles, or can be formed from a metal oxide precursor, for example, by a sol-gel technique.

[0063] Certain embodiments of the closed-pore metal oxide particles exhibit a color in the visible spectrum in a wavelength range selected from the group consisting of 380 nm to 450 nm, 451 nm to 495 nm, 496 nm to 570 nm, 571 nm to 590 nm, 591 nm to 620 nm, 621 nm to 750 nm, 751 nm to 800 nm, and any range defined therebetween (e.g., 496 nm to 620 nm, 450 nm to 750 nm, etc.). In some embodiments, the particles exhibit a color in the ultraviolet spectrum in a wavelength range selected from the group consisting of 100 nm to 400 nm, 100 nm to 200 nm, 200 nm to 300 nm, and 300 nm to 400 nm.

[0064] In certain embodiments, the closed-pore metal oxide particles have one or more of an average diameter of, for example, about 0.5 pm to about 100 pm, an average porosity of greater than about 0.1, greater than about 0.2, greater than about 0.3, greater than about 0.4, greater than about 0.5, greater than about 0.6, greater than about 0.7, greater than about 0.8, or about 0.10 to about 0.80, and an average pore diameter of about 50 nm to about 500 nm. In other embodiments, the particles can have one or more of an average diameter of, for example, about 1 pm to about 75 pm, an average porosity of about 0.10 to about 0.40, and an average pore diameter of about 50 nm to about 800 nm.

[0065] In certain embodiments, the closed-pore metal oxide particles have an average diameter of, for example, about 1 pm to about 75 pm, about 2 pm to about 70 pm, about 3 pm to about 65 pm, about 4 pm to about 60 pm, about 5 pm to about 55 pm, or about 5 pm to about 50 pm; for example, from about any one of about 5 pm, about 6 pm, about 7 pm, about 8 pm, about 9 pm, about 10 pm, about 11 pm, about 12 pm, about 13 pm, about 14 pm, or about 15 pm to about any one of about 16 pm, about 17 pm, about 18 pm, about 19 pm, about 20 pm, about 21 pm, about 22 pm, about 23 pm, about 24 pm, or about 25 pm. Other embodiments have an average diameter of about any one of about 4.5 pm, about 4.8 pm, about 5.1 pm, about 5.4 pm, about 5.7 pm, about 6.0 pm, about 6.3 pm, about 6.6 pm, about 6.9 pm, about 7.2 pm, or about 7.5 pm to about any one of about 7.8 pm, about 8.1 pm, about 8.4 pm, about 8.7 pm, about 9.0 pm, about 9.3 pm, about 9.6 pm, or about 9.9 pm.

[0066] In certain embodiments, the average porosity of the closed-pore metal oxide particles is, for example, any of about 0.10, about 0.12, about 0.14, about 0.16, about 0.18, about 0.20, about 0.22, about 0.24, about 0.26, about 0.28, about 0.30, about 0.32, about 0.34, about 0.36, about 0.38, about 0.40, about 0.42, about 0.44, about 0.46, about 0.48, about 0.50, about 0.52, about 0.54, about 0.56, about 0.58, or about 0.60 to any of about 0.62, about 0.64, about 0.66, about 0.68, about 0.70, about 0.72, about 0.74, about 0.76, about 0.78, about 0.80, or about 0.90. Other embodiments can have an average porosity of any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55, or about 0.57 to any of about 0.59, about 0.61, about 0.63, or about 0.65.

[0067] In some embodiments, the average pore diameter of the closed-pore metal oxide particles is about 3 nm, about 4 nm, about 5 nm, about 10 nm, about 20 nm, or about 25 nm to about 30 nm, about 35 nm, about 40 nm, about 45 nm, or about 50 nm. In other embodiments, the average pore diameter of the metal oxide particles is, for example, any of about 10 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 100 nm, about 120 nm, about 140 nm, about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm, about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm, about 360 nm, about 380 nm, about 400 nm, about 420 nm, or about 440 nm to any of about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm, about 560 nm, about 580 nm, about 600 nm, about 620 nm, about 640 nm, about 660 nm, about 680 nm, about 700 nm, about 720 nm, about 740 nm, about 760 nm, about 780 nm, or about 800 nm. Other embodiments can have an average pore diameter of any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm, or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm, or about 300 nm.

[0068] In certain embodiments, the metal oxide material of the closed-pore metal oxide particles is selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, or a combination thereof. In certain embodiments, the metal oxide comprises titanium dioxide, silicon dioxide, or a combination thereof.

[0069] In certain embodiments, the polymer of the polymeric particles is selected from poly(meth)acrylic acid, poly(meth)acrylate, polystyrene, polyacrylamide, polyvinyl alcohol, polyvinyl acetate, polyester, polyurethane, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, polyvinyl ether, a derivative thereof, a salt thereof, a copolymer thereof, or a combination thereof. For example, the polymer is selected from the group consisting of polymethyl methacrylate, polyethyl methacrylate, poly-n-butyl methacrylate, polystyrene, polychlorostyrene, poly(alpha-methylstyrene), poly(N-hydroxymethyl acrylamide), styrene / methyl methacrylate copolymer, polyalkylated acrylate, polyhydroxy acrylate, polyamino acrylate, polycyano acrylate, polyfluorinated acrylate, poly(N-hydroxymethyl acrylamide), polyacrylic acid, polymethacrylic acid, methyl methacrylate / ethyl acrylate / acrylic acid copolymer, styrene / methyl methacrylate / acrylic acid copolymer, polyvinyl acetate, polyvinyl pyrrolidone, polyvinyl caprolactone, polyvinyl caprolactam, copolymer of methyl methacrylate and [2-(methacryloyloxy)ethyl]trimethylammonium chloride, a derivative thereof, a salt thereof, or a combination thereof.

[0070] In certain embodiments, the weight ratio of the metal oxide particles to the polymeric particles is about 1 / 10, about 2 / 10, about 3 / 10, about 4 / 10, about 5 / 10, about 6 / 10, about 7 / 10, about 8 / 10, about 9 / 10 to about 10 / 9, about 10 / 8, about 10 / 7, about 10 / 6, about 10 / 5, about 10 / 4, about 10 / 3, about 10 / 2, or about 10 / 1. In certain embodiments, the weight ratio of the metal oxide particles to the polymeric particles is 1 / 3, 2 / 3, 1 / 1, or 3 / 2.

[0071] In further embodiments, the closed-cell metal oxide particles can have, for example, about 60.0 wt% to about 99.9 wt% metal oxide, based on the total weight of the closed-cell metal oxide particles. In other embodiments, the closed-cell metal oxide particles comprise about 0.1 wt% to about 40.0 wt% of one or more light absorbers, based on the total weight of the closed-cell metal oxide particles. In other embodiments, the metal oxide is about any of about 60.0 wt%, about 64.0 wt%, about 67.0 wt%, about 70.0 wt%, about 73.0 wt%, about 76.0 wt%, about 79.0 wt%, about 82.0 wt%, or about 85.0 wt% to about any of about 88.0 wt%, about 91.0 wt%, about 94.0 wt%, about 97.0 wt%, about 98.0 wt%, about 99.0 wt%, or about 99.9 wt%, based on the total weight of the closed-cell metal oxide particles.

[0072] In certain embodiments, the closed-cell metal oxide particles are prepared by a method comprising: forming a liquid dispersion of polymer particles and metal oxide particles; forming droplets of the dispersion; drying the droplets to provide polymer template particles comprising polymer and metal oxide; and removing the polymer to provide the closed-cell metal oxide particles. In such embodiments, the resulting closed cells (and thus the enclosed voids formed) are monodisperse.

[0073] In certain embodiments, the closed-cell metal oxide particles are prepared by a method comprising: generating droplets from a particles dispersion comprising metal oxide particles and polymer particles; drying the droplets to provide dried particles comprising a matrix of metal oxide particles with polymer particles embedded therein; and calcining or sintering the dried particles to densify the matrix of metal oxide particles and remove the polymer particles, resulting in the closed-cell metal oxide particles.

[0074] In other embodiments, the closed-cell metal oxide particles are prepared by a method comprising: generating droplets from a particle dispersion comprising polymer particles and a sol-gel of a metal oxide; drying the droplets to provide dried particles comprising a matrix of the metal oxide and the polymer particles; and calcining or sintering the dried particles to remove the polymer particles, resulting in the closed-cell metal oxide particles. An exemplary process is described as follows: The droplets are generated from a particle dispersion comprising polymer particles and a metal oxide precursor (e.g., an aqueous particle dispersion having a pH of 3 to 5). The precursor can be, for example, tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) as a precursor for silica, n-propyl titanate as a precursor for titania, or zirconium acetate as a precursor for zirconium. The droplets are dried to provide dried particles comprising a hydrolyzed precursor of the metal oxide surrounding and coating the polymer particles. The dried particles are then heated to sinter the metal oxide through condensation reactions of the hydrolyzed precursor and to remove the polymer particles through calcination.

[0075] In some embodiments, evaporation of the liquid medium can be performed in the presence of a self-assembled substrate such as a tapered tube or a silicon wafer. In certain embodiments, the dried particle mixture can be recovered, for example, by filtration or centrifugation. In some embodiments, drying comprises microwave irradiation, oven drying, vacuum drying, drying in the presence of a desiccant, or a combination thereof.

[0076] In certain embodiments, droplet formation and collection occur within a microfluidic device. The microfluidic device is, for example, a narrow channel device having a micron-scale droplet junction suitable for producing uniformly sized droplets, with the channel connected to a collection vessel. For example, the microfluidic device contains a droplet junction having a channel width of about 10 μιη to about 100 μιη. The device is, for example, made of polydimethylsiloxane (PDMS) and can be fabricated, for example, by soft lithography. The emulsion can be prepared within the device by pumping the aqueous dispersed phase and the oily continuous phase into the device at a specified rate where mixing occurs to provide emulsion droplets. Alternatively, an oil-in-water emulsion can be utilized. The continuous oil phase comprises, for example, an organic solvent, a silicone oil, or a fluorinated oil. As used herein, "oil" refers to an organic phase (e.g., an organic solvent) that is immiscible with water. Organic solvents include hydrocarbons such as heptane, hexane, toluene, xylene, and the like.

[0077] In certain embodiments having droplets, the droplets are formed from a microfluidic device. The microfluidic device can contain a droplet junction having a channel width of, for example, about any of 10 μιη, 15 μιη, 20 μιη, 25 μιη, 30 μιη, 35 μιη, 40 μιη, or 45 μιη to about any of 50 μιη, 55 μιη, 60 μιη, 65 μιη, 70 μιη, 75 μιη, 80 μιη, 85 μιη, 90 μιη, 95 μιη, or 100 μιη.

[0078] In certain embodiments, the generation and drying of the droplets is performed using a spray drying method. Figure 3 A schematic diagram of an exemplary spray drying system 300 used in accordance with various embodiments of the present disclosure is shown. In certain embodiments of the spray drying technique, a feed 302 of a liquid solution or dispersion is fed (e.g., pumped) to an atomizing nozzle 304 associated with a compressed gas inlet through which gas 306 is injected. The feed 302 is pumped through the atomizing nozzle 304 to form droplets 308. The droplets 308 are surrounded by a preheated gas in an evaporation chamber 310, causing the solvent to evaporate to produce dried particles 312. The dried particles 312 are carried by a drying gas through a cyclone separator 314 and deposited in a collection chamber 316. The gas includes nitrogen and / or air. In embodiments of the exemplary spray drying method, the liquid feed contains an aqueous or oil phase, metal oxide, and polymeric particles. The dried particles 312 comprise a self-assembled structure of each polymeric particle surrounded by metal oxide particles.

[0079] Air can be considered as a continuous phase with a dispersed liquid phase (liquid-in-gas emulsion). In certain embodiments, the spray drying comprises an inlet temperature of any of about 100°C, about 105°C, about 110°C, about 115°C, about 120°C, about 130°C, about 140°C, about 150°C, about 160°C, or about 170°C to any of about 180°C, about 190°C, about 200°C, about 210°C, about 215°C, or about 220°C. In some embodiments, a pump rate (feed flow rate) of any of about 1 mL / min, about 2 mL / min, about 5 mL / min, about 6 mL / min, about 8 mL / min, about 10 mL / min, about 12 mL / min, about 14 mL / min, or about 16 mL / min, about 18 mL / min, about 20 mL / min, about 22 mL / min, about 24 mL / min, about 26 mL / min, about 28 mL / min, or about 30 mL / min is utilized.

[0080] In some embodiments, a vibrating nozzle technique can be employed. In such a technique, a liquid dispersion is prepared and then droplets are formed and dropped into a continuous phase bath. The droplets are then dried. Vibrating nozzle equipment is available from and comprises, for example, a syringe pump and a pulsation unit. The vibrating nozzle equipment can also comprise a pressure regulating valve.

[0081] In certain embodiments, the polymer removal can be performed, for example, by calcination, pyrolysis, or with a solvent (solvent removal). In some embodiments, the calcination is performed at a temperature of at least about 200 °C, at least about 500 °C, at least about 1000 °C, from about 200 °C to about 1200 °C, or from about 200 °C to about 700 °C. The calcination can be for a suitable time, for example, from about 0.1 hour to about 12 hours or from about 1 hour to about 8.0 hours. In other embodiments, the calcination can be for at least about 0.1 hour, at least about 1 hour, at least about 5 hours, or at least about 10 hours. In other embodiments, the calcination can be for a time of from any of about 200 °C, about 350 °C, about 400 °C, 450 °C, about 500 °C, or about 550 °C to any of about 600 °C, about 650 °C, about 700 °C, or about 1200 °C for any of about 0.1 hour, about 1 hour, about 1.5 hours, about 2.0 hours, about 2.5 hours, about 3.0 hours, about 3.5 hours, or about 4.0 hours to any of about 4.5 hours, about 5.0 hours, about 5.5 hours, about 6.0 hours, about 6.5 hours, about 7.0 hours, about 7.5 hours, about 8.0 hours, or about 12 hours. When the polymer is removed in this process, the closed cells that remain after calcination will substantially maintain the void volume array.

[0082] In certain embodiments, the ratio of the particle size of the metal oxide particles to the polymer particles is from 1 / 50 to 1 / 5 (e.g., 1 / 10).

[0083] In certain embodiments, the metal oxide particles have an average diameter of from about 1 nm, about 5 nm, about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm, or about 60 nm to about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, about 100 nm, about 105 nm, about 110 nm, about 115 nm, or about 120 nm. In other embodiments, the matrix nanoparticles have an average diameter of from about 5 nm to about 150 nm, from about 50 nm to about 150 nm, or from about 100 nm to about 150 nm.

[0084] In certain embodiments, the polymeric particles have an average diameter of from about 50 nm to about 990 nm. In other embodiments, the particles have an average diameter of from any of about 50 nm, about 75 nm, about 100 nm, about 130 nm, about 160 nm, about 190 nm, about 210 nm, about 240 nm, about 270 nm, about 300 nm, about 330 nm, about 360 nm, about 390 nm, about 410 nm, about 440 nm, about 470 nm, about 500 nm, about 530 nm, about 560 nm, about 590 nm, or about 620 nm to any of about 650 nm, about 680 nm, about 710 nm, about 740 nm, about 770 nm, about 800 nm, about 830 nm, about 860 nm, about 890 nm, about 910 nm, about 940 nm, about 970 nm, or about 990 nm.

[0085] In certain embodiments, removing the polymeric particles comprises calcination, pyrolysis, or solvent removal. Calcination of the polymeric particles can be performed, for example, at a temperature of from about 300 °C to about 800 °C for a time of from about 1 hour to about 8 hours.

[0086] In certain embodiments, the closed-cell metal oxide particles comprise primarily metal oxide, i.e., they can consist essentially of or consist of metal oxide. Advantageously, depending on the particle composition, relative size, and shape of the metal oxide particles used, bulk samples of the closed-cell metal oxide particles can exhibit a color observable by the human eye, can appear white, or can exhibit a characteristic in the UV spectrum. Light absorbers can also be present in the particles, which can provide a more saturated observable color. Absorbers include inorganic and organic materials, such as broadband absorbers like carbon black. The absorbers can be added, for example, by physically mixing the particles and absorbers together or by including the absorbers in the droplets to be dried. In certain embodiments, the closed-cell metal oxide particles can exhibit no observable color without the addition of light absorbers and an observable color with the addition of light absorbers.

[0087] The closed-cell metal oxide particles described herein can exhibit an angle-dependent color or an angle-independent color. By "angle-dependent" color is meant that the color observed depends on the angle of the incident light on the sample or the angle between the observer and the sample. By "angle-independent" color is meant that the color observed is substantially independent of the angle of the incident light on the sample or the angle between the observer and the sample.

[0088] For example, angle-dependent color can be achieved by using monodisperse polymeric particles. Angle-dependent color can also be achieved when the steps of the drying droplets are performed slowly, allowing the particles to become ordered. Angle-independent color can be achieved when the steps of the drying droplets are performed quickly, without allowing the particles to become ordered.

[0089] The following embodiments can be used to achieve angle-dependent color resulting from ordered pores left behind after removal of polymers. As a first example embodiment of angle-dependent color, monodisperse and spherical polymer particles are embedded in metal oxide particles, and the metal oxide particles are subsequently densified and the polymers are removed. The metal oxide particles can be spherical or non-spherical. As a second example embodiment of angle-dependent color, two or more polymer particles that are overall monodisperse and spherical are embedded in metal oxide particles, and the metal oxide particles are subsequently densified and the polymers are removed. The angle-dependent color is achieved independent of the polydispersity and shape of the host particles.

[0090] The following embodiments can be used to achieve angle-independent color resulting from disordered pores left behind after removal of polymers. As a first example embodiment of angle-independent color, polydisperse polymer particles are embedded in metal oxide particles, and the metal oxide particles are subsequently densified and the polymers are removed.

[0091] As a second example embodiment of angle-independent color, two different sizes of polymer particles (i.e., bimodal monodisperse polymer particles) are embedded in metal oxide particles, and the metal oxide particles are subsequently densified and the polymers are removed. The metal oxide particles can be spherical or non-spherical.

[0092] As a third example embodiment of angle-independent color, two different sizes and polydisperse spherical polymer particles are embedded in metal oxide particles, and the metal oxide particles are subsequently densified and the polymers are removed.

[0093] The angle-independent color is achieved independent of the polydispersity and shape of the host particles.

[0094] Any of the embodiments exhibiting angle-dependent color or angle-independent color can be modified to exhibit whiteness or effects (e.g., reflectivity, absorptivity) in the ultraviolet spectrum.

[0095] In some embodiments, the metal oxide particles can comprise a combination of different types of particles. For example, the metal oxide particles can be a mixture of two different metal oxides (i.e., a discrete distribution of metal oxide particles), such as a mixture of alumina particles and silica particles, where each particle is characterized by the same or similar size distribution.

[0096] In some embodiments, the metal oxide particles can comprise more complex compositions and / or morphologies. For example, the metal oxide particles can comprise particles such that each individual particle comprises two or more metal oxides (e.g., silica-titania particles). Such particles can comprise, for example, a mixture of two or more metal oxides.

[0097] In some embodiments, the metal oxide particles and / or the polymer particles can comprise surface functionalization. An example of surface functionalization is a silane coupling agent (e.g., silane functionalized silica). In some embodiments, the metal oxide particles are surface functionalized prior to self-assembly and densification. In some embodiments, the closed-cell metal oxide particles are surface functionalized after densification. In some embodiments, the surface functionalization can be selected to impart a net positive surface charge or a net negative surface charge to the particles when dispersed in aqueous solution.

[0098] As used herein, particle size is synonymous with particle diameter and is determined, for example, by scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Average particle size is synonymous with D50, meaning that half the population lies above this point and half below. Particle size refers to primary particles. Particle size can be measured by laser scattering techniques with dispersions or dry powders.

[0099] Mercury porosimetry analysis can be used to characterize the porosity of the particles. Mercury porosimetry measures the application of a controlled pressure to a sample immersed in mercury. The external pressure applied causes the mercury to penetrate into the voids / pores of the material. The magnitude of pressure required to invade the voids / pores is inversely proportional to the size of the voids / pores. Mercury porosimetry uses the Washburn equation to generate volume and pore size distributions from the pressure versus intrusion data generated by the instrument. The porosity of the closed-cell metal oxide particles as reported herein is calculated as the ratio of unoccupied space to total particle volume. For example, porous silica particles containing voids / pores with an average size of 165 nm have an average porosity of 0.8.

[0100] Illustrative Examples

[0101] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the embodiments disclosed herein, and are not intended to limit the scope of what can be claimed. Various changes in the methodology of the embodiments described herein, and equivalents thereof, may

[0102] Example 1: Preparation of closed-cell silica particles by microfluidics

[0103] An aqueous dispersion of positively charged poly(alk)acrylate nanoparticles was diluted to 1 wt% with deionized water and 3 wt% of negatively charged silica nanoparticles were added. The mixture was sonicated for 30 seconds to prevent clumping. The aqueous nanoparticle dispersion and the oil phase (a continuous oil phase containing 2 wt% of polyethylene glycol-co-perfluoropolyester surfactant in a fluorinated oil) were each injected into a microfluidic device with a 50 pm droplet junction by syringe pumps. The system was allowed to equilibrate until monodisperse droplets were produced. The droplets were collected in a container.

[0104] The collected droplets were dried in an oven at 50 °C for 4 hours. The dried powder was calcined on a silicon wafer, heated from room temperature to 500 °C over a 4 hour period, held at 500 °C for 2 hours, and cooled back to room temperature over a 4 hour period. This process resulted in monodisperse closed-cell silica particles with a diameter of 15 microns.

[0105] Figure 4 SEM images of closed-cell metal oxide particles produced according to the microfluidic process are shown (top image), as well as a cross-section of the closed-cell metal oxide particles (bottom image), which shows that the internal structure comprises an array of closed-cell metal oxide shells, each shell encompassing a relatively monodisperse and ordered void.

[0106] Example 2: Closed-cell silica particle encapsulated media - inaccessible void volume

[0107] The powder product from Example 1 was dispersed in mineral oil at a mass concentration of 3 wt.%. Porous silica particles were also dispersed in mineral oil at the same concentration for comparison. Figure 5 Photographs of (a) the powder product of closed-cell silica particles, (b) closed-cell silica particles in mineral oil, (c) the powder product of porous silica particles, and (d) porous silica particles in mineral oil are shown. The suspension of closed-cell silica particles exhibited a hazy appearance. The closed-cell silica particles did not disappear in mineral oil, which has a refractive index of 1.46 to 1.47, demonstrating that the closed-cell morphology prevents media from infiltrating the enclosed voids. In contrast, the suspension of porous silica particles exhibited a clear appearance. The porous particles disappeared after the oil infiltrated the voids due to the refractive index match between the silica (which has a refractive index of about 1.47) and the mineral oil.

[0108] Example 3: Closed-cell silica particles with ordered voids produced by spray drying

[0109] An aqueous suspension of positively charged spherical polymeric nanoparticles (a copolymer of methyl methacrylate and 2-(methacryloyloxy)ethyl] trimethylammonium chloride nanoparticles, average diameter 254 nm) and negatively charged silica nanoparticles (average diameter 7 nm) was prepared. The polymeric nanoparticles were present at 1.8 wt.% and the silica nanoparticles were present at 0.6 wt.% by weight of the aqueous suspension (3: 1 weight ratio of polymeric nanoparticles to metal oxide nanoparticles). The aqueous suspension was spray dried using a laboratory scale spray dryer under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, a spray gas pressure of 40 mm, a suction rate of 100%, and a flow rate of 30% (about 10 mL / min). A laboratory scale spray dryer, under an inert atmosphere (nitrogen), at an inlet temperature of 100 °C, a spray gas pressure of 40 mm, a suction rate of 100%, and a flow rate of 30% (about 10 mL / min), was used to spray dry the aqueous suspension.

[0110] The spray-dried powder was removed from the collection chamber of the spray-dryer and spread onto a silicon wafer for sintering. The spray-dried powder was then calcined in a muffle furnace using an intermittent sintering process to sinter and densify the silica nanoparticles and remove the polymer to produce the closed-cell silica particles. The heating parameters were as follows: the particles were heated from room temperature to 550 °C over a period of 5 hours, held at 550 °C for 2 hours, and then cooled back to room temperature over a period of 3 hours.

[0111] Figure 6 An SEM image of the closed-cell silica particles produced according to the spray-drying process is shown (top image), as well as a cross-section of the closed-cell silica particles (bottom image), which shows that the internal structure comprises an array of closed-cell silica shells, each shell encompassing a monodisperse and ordered void.

[0112] Example 4: Closed-cell silica particles containing a light absorber

[0113] The product of Example 1 was physically mixed with an aqueous dispersion of carbon black or carbon black powder at different weight levels. The resulting closed-cell silica particles contained carbon black at a content of 0.5 wt.%, 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, and 5 wt.% by total weight of the particles.

[0114] Example 5: Visible color in bulk sample

[0115] Closed-cell silica particles of Example 1 (0.5 mg) were evenly distributed in a 20-mL clear glass vial with a 6 cm 2 The sample exhibited a noticeable blue color observable by the human eye. Figure 7 is a UV-vis spectrum of the sample, which shows a reflection peak at 440 nm corresponding to blue color.

[0116] A sample of closed-cell silica particles was produced in a similar manner as Example 1, except that the weight ratio of polymer to silica was 2:1. The sample exhibited a noticeable green color observable by the human eye. Figure 8 is a UV-vis spectrum of the sample, which shows a reflection peak at 520 nm corresponding to green color.

[0117] Example 6: Closed-cell silica particles demonstrating UV attenuation

[0118] A sample of closed-cell silica particles was produced in a similar manner as Example 1, except that PMMA nanoparticles with a diameter of 140 nm were used and the weight ratio of polymer to silica was 3:1. The sample exhibited attenuation in the UV range. The closed-cell silica particles showed attenuation in the UV range. The UV attenuation of the silica nanoparticles was used as a control sample, and the relatively low attenuation value indicates that the UV attenuation of the closed-cell silica particles is not coming from the silica nanoparticles.

[0119] Figure 9 Figure 2 is a graph showing the relative attenuation values of closed-pore titania particles and titania nanoparticles in the UV range.

[0120] Example 7: Closed-pore titania particles

[0121] An aqueous suspension of negatively charged spherical polystyrene nanoparticles (average diameter 197 nm) and positively charged titania nanoparticles (average diameter 15 nm) was prepared. The polystyrene nanoparticles were present at 1.8 wt% and the titania nanoparticles were present at 1.2 wt% by weight of the aqueous suspension (3:2 weight ratio of polymer nanoparticles to metal oxide nanoparticles). The aqueous suspension was spray dried using a laboratory scale spray dryer under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, a spray gas pressure of 55 mm, a suction rate of 100% and a flow rate of 30% (approximately 10 mL / min). An aqueous suspension of negatively charged spherical polystyrene nanoparticles (average diameter 197 nm) and positively charged titania nanoparticles (average diameter 15 nm) was prepared. The polystyrene nanoparticles were present at 1.8 wt% and the titania nanoparticles were present at 1.2 wt% by weight of the aqueous suspension (3:2 weight ratio of polymer nanoparticles to metal oxide nanoparticles). The aqueous suspension was spray dried using a laboratory scale spray dryer under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, a spray gas pressure of 55 mm, a suction rate of 100% and a flow rate of 30% (approximately 10 mL / min).

[0122] The spray dried powder was removed from the collection chamber of the spray dryer and spread onto a silicon wafer for sintering. The spray dried powder was then calcined in a muffle furnace using an intermittent sintering process to sinter and densify the titania nanoparticles and remove the polymer to produce closed-pore titania particles. The heating parameters were as follows: the particles were heated from room temperature to 300 °C over a period of 4 hours, held at 300 °C for 6 hours, then heated to 550 °C over a period of 2 hours, held at 550 °C for 2 hours, and cooled back to room temperature over a period of 4 hours.

[0123] Figure 10 SEM images of the closed-pore titania particles produced according to the spray drying process are shown (lower image), as well as a cross-section of the closed-pore titania particles (upper image) which shows that the internal structure comprises an array of closed-pore titania shells, each shell encompassing a relatively monodisperse void.

[0124] Example 8: Closed-pore silica particles prepared by sol-gel process

[0125] An aqueous suspension of positively charged spherical polymer nanoparticles (copolymer of methyl methacrylate and 2-(methacryloyloxy)ethyl trimethylammonium chloride nanoparticles, average diameter 254 nm) and a silica precursor, tetramethyl orthosilicate (TMOS), was mixed at a pH range of 2 to 5. The polymer nanoparticles were present at 1.8 wt% and the TMOS was present at 3.6 wt% by weight of the aqueous suspension (1 :3 weight ratio of polymer nanoparticles to metal oxide nanoparticles). The aqueous suspension was spray dried using a laboratory scale spray dryer under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, a spray gas pressure of 55 mm, a suction rate of 100% and a flow rate of 30% (approximately 10 mL / min). A laboratory scale spray dryer was used to spray dry the aqueous suspension under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, 40 mm spray gas pressure, 100% aspirator rate, and 30% flow rate (about 10 mL / min).

[0126] The spray dried powder was removed from the collection chamber of the spray dryer and spread onto a silicon wafer for sintering. The spray dried powder was then calcined in a muffle furnace using an intermittent sintering process to convert the silica precursor to silica nanoparticles and to densify the silica, and to remove the polymer to produce closed cell silica particles. The heating parameters were as follows: heat the particles from room temperature to 200 °C over a period of 3 hours, hold at 200 °C for 2 hours, then heat to 550 °C over a period of 2 hours, hold at 550 °C for 2 hours, and cool back to room temperature over a period of 3 hours.

[0127] Figure 11 SEM images of the product produced in Example 8 are shown.

[0128] Example 9: Closed cell silica particles with disordered voids

[0129] Aqueous suspensions of positively charged spherical polymer nanoparticles (copolymers of methyl methacrylate and 2-(methacryloyloxy)ethyl trimethylammonium chloride nanoparticles) and negatively charged silica nanoparticles (average diameter of 7 nm) of two different sizes (254 nm and 142 nm in diameter, respectively) were prepared. The polymer nanoparticles were present in total at 1.8 wt% (0.9 wt% of each) and the silica nanoparticles were present at 0.6 wt% by weight of the aqueous suspension. The A laboratory scale spray dryer was used to spray dry the aqueous suspension under an inert atmosphere (nitrogen) at an inlet temperature of 100 °C, 40 mm spray gas pressure, 100% aspirator rate, and 30% flow rate (about 10 mL / min).

[0130] The spray dried powder was removed from the collection chamber of the spray dryer and spread onto a silicon wafer for sintering. The spray dried powder was then calcined in a muffle furnace using an intermittent sintering process to convert the silica precursor to silica nanoparticles and to densify the silica, and to remove the polymer to produce closed cell silica particles. The heating parameters were as follows: heat the particles from room temperature to 200 °C over a period of 3 hours, hold at 200 °C for 2 hours, then heat to 550 °C over a period of 2 hours, hold at 550 °C for 2 hours, and cool back to room temperature over a period of 3 hours.

[0131] The closed cell silica particles (0.5 mg) were evenly distributed in a 20-mL clear glass vial with a 6 cm 2 The sample exhibited a human eye observable angle independent blue color.

[0132] In the foregoing description, numerous specific details are set forth to provide a thorough understanding of embodiments of the present disclosure. One or more specific features, structures, materials, or characteristics described herein can be combined with any or all other specific features, structures, materials, or characteristics described herein. As used in this application, the word "or" is intended to mean an inclusive "or" rather than an exclusive "or." That is, unless specified otherwise, or clear from context, "X includes A or B" means that X includes A, X includes B, or X includes both A and B. Further, the word "a" or "an" as used herein means "one or more" unless otherwise analyzed or clear from context to indicate a singular noun only. The word "comprising" as used in this application means "including, but not limited to."

[0133] As used in this application, the word "or" is intended to mean an inclusive "or" rather than an exclusive "or." That is, unless specified otherwise, or clear from context, "X includes A or B" means that X includes A, X includes B, or X includes both A and B. Further, the word "a" or "an" as used herein means "one or more" unless otherwise analyzed or clear from context to indicate a singular noun only. The word "comprising" as used in this application means "including, but not limited to."

[0134] Reference throughout this specification to "an embodiment", "certain embodiments", or "one embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Therefore, appearances of the phrases "an embodiment", "certain embodiments", or "one embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, and such phrases, in

[0135] It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Therefore, the scope of the present disclosure should be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled.

Claims

1. A method for preparing closed-cell metal oxide particles, the method comprising: Droplets are generated from a particulate dispersion comprising a first particle containing a polymer material and a second particle containing a metal oxide material, wherein the average diameter of the first particle is 50 nm to 500 nm and the average diameter of the second particle is 1 nm to 120 nm. Drying the droplets to provide dried particles comprising an array of the first particles, wherein each of the first particles is coated with the second particle layer; and The dried particles are calcined or sintered, wherein the calcination or sintering densifies the metal oxide material and removes the polymer material to produce the closed-cell metal oxide particles, each closed-cell metal oxide particle comprising a metal oxide matrix defining a closed-cell array, each closed-cell encapsulating medium being inaccessible to the pore volume, and wherein the outer surface of the closed-cell metal oxide particles is defined by their respective closed-cell arrays. Wherein the first particle comprises a net positively charged surface, and wherein the second particle comprises a net negatively charged surface, or The first particle contains a net negatively charged surface, and the second particle contains a net positively charged surface.

2. The method according to claim 1, wherein the closed-hole array is an ordered array.

3. The method according to claim 1, wherein the closed-hole array is a disordered array.

4. The method of claim 1, wherein the second particle layer is formed on the first particle by surface charge driving.

5. The method of claim 2, wherein the second particle layer is formed on the first particle by surface charge driving.

6. The method of claim 3, wherein the second particle layer is formed on the first particle by surface charge drive.

7. The method according to any one of claims 1 to 6, wherein the polymer material comprises a polymer selected from copolymers of poly(meth)acrylic acid, poly(meth)acrylate, polystyrene, polyacrylamide, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, methyl methacrylate, and [2-(methacryloyloxy)ethyl]trimethylammonium chloride.

8. The method according to any one of claims 1 to 6, wherein the metal oxide material comprises a metal oxide selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

9. The method of claim 7, wherein the metal oxide material comprises a metal oxide selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

10. The method of claim 8, wherein the metal oxide material comprises silicon dioxide.

11. The method of claim 9, wherein the metal oxide material comprises silicon dioxide.

12. The method according to claim 1, wherein the average diameter of the closed-pore metal oxide particles is from 0.5 μm to 100 μm.

13. The method according to any one of claims 1 to 6 and 9 to 12, wherein the droplets are generated using a microfluidic method.

14. The method of claim 7, wherein the droplets are generated using a microfluidic method.

15. The method of claim 8, wherein the droplets are generated using a microfluidic method.

16. The method according to any one of claims 1 to 6 and 9 to 12, wherein the generation and drying of the droplets are carried out using spray drying.

17. The method of claim 7, wherein the droplets are generated and dried using a spray drying method.

18. The method of claim 8, wherein the droplets are generated and dried using a spray drying method.

19. The method according to any one of claims 1 to 6 and 9 to 12, wherein the droplets are generated using a vibrating nozzle.

20. The method of claim 7, wherein a vibrating nozzle is used to generate the droplets.

21. The method of claim 8, wherein a vibrating nozzle is used to generate the droplets.

22. The method according to any one of claims 1 to 6, 9 to 12, 14, 15, 17, 18, 20 and 21, wherein drying the droplets comprises evaporation, microwave radiation, oven drying, vacuum drying, drying in the presence of a desiccant, or a combination thereof.

23. The method according to any one of claims 1 to 6, 9 to 12, 14, 15, 17, 18, 20 and 21, wherein the particulate dispersion is an aqueous particulate dispersion.

24. The method of claim 22, wherein the particulate dispersion is an aqueous particulate dispersion.

25. The method according to any one of claims 1 to 6, 9 to 12, 14, 15, 17, 18, 20, 21 and 24, wherein the weight ratio of the first particle to the second particle is 1 / 10 to 10 / 1.

26. The method of claim 25, wherein the weight ratio of the first particle to the second particle is 2 / 3, 1 / 1, 3 / 2, or 3 / 1.

27. The method according to any one of claims 1 to 6, 9 to 12, 14, 15, 17, 18, 20, 21, 24 and 26, wherein the particle size ratio of the second particle to the first particle is 1 / 50 to 1 / 5.

28. The method of claim 25, wherein the particle size ratio of the second particle to the first particle is 1 / 50 to 1 / 5.

29. A method for preparing closed-cell metal oxide particles, the method comprising: Droplets are generated from a particulate dispersion comprising polymer particles in a sol-gel matrix of a metal oxide material, the polymer particles comprising a polymer material, wherein the average diameter of the polymer particles is 50 nm to 500 nm; Drying the droplets to provide dried particles comprising an array of polymer particles, each of which is coated with the sol-gel matrix; and The dried particles are calcined or sintered to obtain the closed-cell metal oxide particles, wherein the calcination or sintering removes the polymer material and densifies the metal oxide material to produce the closed-cell metal oxide particles, each closed-cell metal oxide particle comprising a metal oxide matrix defining a closed-cell array, each closed-cell encapsulating medium being inaccessible to the pore volume, and wherein the outer surface of the closed-cell metal oxide particles is defined by their respective closed-cell arrays. The polymer particles comprise a net positively charged surface, and the sol-gel matrix of the metal oxide material comprises a net negatively charged surface, or The polymer particles contain a net negatively charged surface, and the sol-gel matrix of the metal oxide material contains a net positive charge.

30. Closed-cell metal oxide particles prepared by any one of claims 1 to 29.

31. A closed-cell metal oxide particle comprising a metal oxide matrix defining a closed-cell array, each closed-cell encapsulating a medium-inaccessible void volume, wherein the outer surface of the closed-cell metal oxide particle is defined by the closed-cell array, wherein the average diameter of the void volume is from 50 nm to 500 nm, and the average diameter of the closed-cell metal oxide particle is in the range of 0.5 μm to 100 μm.

32. The closed-cell metal oxide particles according to claim 31, wherein the closed-cell array is an ordered array.

33. The closed-pore metal oxide particles according to claim 31, wherein the closed-pore array is a disordered array.

34. Closed-cell metal oxide particles according to any one of claims 31 to 33, wherein the metal oxide matrix comprises a metal oxide selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

35. The closed-cell metal oxide particles of claim 34, wherein the metal oxide matrix comprises silicon dioxide.

36. Closed-cell metal oxide particles according to any one of claims 31 to 33 and 35, wherein at least part of them are derived from polymer particles with an average diameter of 50 nm to 500 nm.

37. The closed-cell metal oxide particles of claim 34, wherein at least part of them are derived from polymer particles with an average diameter of 50 nm to 500 nm.

38. Closed-cell metal oxide particles according to any one of claims 31 to 33, 35 and 37, wherein at least a portion is derived from metal oxide particles with an average diameter of 1 nm to 120 nm.

39. The closed-cell metal oxide particles of claim 34, wherein at least a portion is derived from metal oxide particles with an average diameter of 1 nm to 120 nm.

40. The closed-cell metal oxide particles of claim 36, wherein at least a portion is derived from metal oxide particles with an average diameter of 1 nm to 120 nm.

41. Closed-cell metal oxide particles according to any one of claims 31 to 33 and 35, wherein the particles are derived from metal oxide precursors selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

42. The closed-cell metal oxide particles according to claim 34, wherein the particles are derived from metal oxide precursors selected from silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, cerium dioxide, iron oxide, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.

43. Closed-cell metal oxide particles according to any one of claims 30 to 33, 35, 37, 39, 40 and 42, further comprising a light absorber.

44. The closed-cell metal oxide particles of claim 43, wherein the light absorber is present in an amount of 0.1% to 40.0% by weight.

45. The closed-cell metal oxide particles of claim 43, wherein the light absorber comprises carbon black.

46. ​​The closed-cell metal oxide particles of claim 44, wherein the light absorber comprises carbon black.

47. The closed-cell metal oxide particles according to claim 43, wherein the light absorber comprises one or more types of ions.

48. The closed-cell metal oxide particles according to claim 44, wherein the light absorber comprises one or more types of ions.

49. A composition comprising a plurality of closed-cell metal oxide particles according to any one of claims 31 to 48.

50. The composition of claim 49, wherein the average diameter of the closed-pore metal oxide particles is in the range of 0.5 μm to 100 μm.

51. The composition of claim 49, further comprising a substrate on which the closed-pore metal oxide particles are disposed.

52. The composition of claim 50, further comprising a substrate on which the closed-cell metal oxide particles are disposed.

53. The composition according to any one of claims 49 to 52, wherein the composition is an aqueous formulation or an oil-based formulation.

54. The composition according to any one of claims 49 to 52, wherein the composition is an ink, coating formulation, food, plastic, cosmetic formulation, or material for medical or safety applications.

55. A bulk composition exhibiting whiteness, non-whiteness, or effects in the ultraviolet spectrum, said bulk composition comprising a plurality of closed-cell metal oxide particles according to any one of claims 30 to 48, said effects including reflectance or absorptivity.

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