CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No.

62/316,772, filed April 1, 2016 and titled "Formation of High Quality Titania, Alumina, and Other Metal Oxide Templated Materials Through Coassembly," the entire contents of which are hereby incorporated by reference in their entirety.

INCORPORATION BY REFERENCE

[0002] All patents, patent applications and publications cited herein are hereby incorporated by reference in their entirety in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described herein.

FIELD OF THE INVENTION

[0003] The present application relates to the formation of high-quality templated materials. More particularly, the present application relates to mixed amorphous-crystalline precursors for templating metal oxide structures, for example inverse opals, using co- assembly.

BACKGROUND

[0004] Inverse replicas of opals, or "inverse opals," comprise regular arrangements of pores that collectively exhibit light reflective properties and methods of making the same.

SUMMARY

[0005] In accordance with certain embodiments, a co-assembly method for synthesizing templated materials, such as inverse photonic structures, is described. The method includes combining an onium compound with a sol-gel precursor to form metal oxide (MO) nanocrystals, where each MO nanocrystal has crystalline and amorphous content (i.e., a crystalline phase and an amorphous phase). The MO nanocrystals are combined with templating particles to form a suspension. The combining can comprise arranging the templating particles into a direct opal structure. A solvent is evaporated from the suspension to form an intermediate or compound product. The intermediate or compound product can then be calcined to produce a templated material, such as an inverse opal or other inverse structure. The templated material can be a photonic structure.

[0006] In accordance with certain embodiments, the onium compound is a quarternary ammonium salt, for example an alkyl ammonium hydroxide such as tetramethyl ammonium hydroxide (TMAH). A molar ratio of the onium compound to the sol-gel precursor can be at least about 0.05, for example between about 0.3 and about 1.85. Combining the onium compound with the sol-gel precursor can be performed in a liquid, which can comprise at least one of an aqueous solvent, an organic solvent, and a mixed solvent.

[0007] In accordance with certain embodiments, the sol-gel precursor includes titanium isopropoxide (TIP), aluminum isopropoxide (AIP), and/or zirconium 1-propoxide (ZIP).

[0008] In accordance with certain embodiments, the metal oxide nanocrystals comprise at least one of: titania, zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noble metal oxide, platinum group metal oxide, hafnia, molybdenum oxide, tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides, vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanum oxides, ceria, thorium oxides, uranium oxides, other rare earth oxides, and combinations thereof.

[0009] In accordance with certain embodiments, the suspension has a final solid content in a range of about 0.05% to about 10% by weight, or of up to about 20% by weight. The suspension can be dispersed, for example, within a droplet having a diameter that is between about 0.1 μπι and about 10 mm, or between about 0.5 μπι and about 5 mm, or between about 1 μπι and about 1 mm.

[0010] In accordance with certain embodiments, the compound product is a thin film deposited onto a surface, such as a surface of a substrate. In some such embodiments, the method also includes suspending the substrate in the suspension prior to the solvent evaporation.

[0011] In accordance with certain embodiments, the templating particles include a colloidal suspension of spherical, elongated, concave, amorphous, or facetted particles made from polymer, metal, metal oxide, supramolecular aggregates, crystals of organic inorganic and organometallic compounds, or salts. For example, the templating particles can include a colloidal suspension of polymeric spheres. In some implementations, the templating particles can also comprise metal nanoparticles. [0012] In accordance with certain embodiments, the photonic structure comprises titania. The photonic structure can be formed as a film, a brick, or a spherical particle. The photonic structure can be crack-free for at least 10,000 repeat units thereof.

[0013] In accordance with certain embodiments, the method also includes combining the metal oxide nanocrystals and the templating particles with functional nanoparticles, which may include one or more metals (e.g., metal nanoparticles).

[0014] In accordance with certain embodiments, the inverse opal comprises at least one of: a titania-gold nanoparticle inverse opal film, a titania-platinum nanoparticle inverse opal film, a titania-palladium nanoparticle inverse opal film, titania-gold nanoparticle inverse opal photonic bricks, and spherical titania-gold nanoparticle inverse opal particles.

[0015] Templated structures described herein can be useful in structural pigments, cosmetic products, pharmaceutical products, edible products, drug delivery

device/mechanisms, fluidic devices, cooling devices, tissue engineering, membranes, sensors, filtration, sorption/desorption, support media, acoustic devices, batteries, fuel cells, photoactive catalysts, catalytic mediums or supports, coherent scattering media, patterned structure fabrication, light emitters, random lasing or other optical applications, such as smart displays or other electrochromic materials.

[0016] In some implementations, the coassembly method includes combining an alkyl ammonium compound with a metal alkoxide to form MO nanocrystals, e.g., titanium dioxide (T1O2) nanocrystals, which are combined with a polymeric colloid to form a suspension. Solvent is evaporated from the suspension to form an intermediate product, which then undergoes calcination to produce an inverse opal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

[0018] FIG. 1 is a diagram of a coassembly method for synthesizing photonic structures utilizing metal oxide nanocrystals, in accordance with certain embodiments.

[0019] FIG. 2A illustrates a titanium(IV) bis(ammonium lactato) dihydroxide

(TiBALDH) sol-gel precursor and the resulting microstructure (schematic and scanning electron microscope (SEM) image) when the TiBALDH is used to co-assemble an inverse opal.

[0020] FIG. 2B illustrates a crystalline metal oxide (MO) nanoparticle and the resulting microstructure (schematic and scanning electron microscopy (SEM) image) when the MO nanoparticle is used to co-assemble an inverse opal.

[0021] FIG. 2C illustrates a crystalline MO nanocrystal with amorphous phase and the resulting microstructure (schematic and scanning electron microscope (SEM) image) when the crystalline MO nanocrystal with amorphous phase is used to synthesize an inverse opal, in accordance with certain embodiments.

[0022] FIG. 2D is a plot showing the proportion of amorphous phase to crystalline phase in metal oxide nanocrystals, versus the ratio (RTIP/TMAH) of titanium isopropoxide (TIP) to tetramethylammonium hydroxide (TMAH) used to prepare them, in accordance with some embodiments.

[0023] FIGS. 3A-3C are transition electron microscope (TEM) images of as-synthesized titania nanocrystal precursors made with different ratios of titanium precursor, in accordance with certain embodiments.

[0024] FIGS. 4A-4B are SEM images of large-area, crack-free titania inverse opal films using 420 nm PS colloids on (ITO coated PET) conductive plastic substrate using

nanocrystalline titania, in accordance with certain embodiments.

[0025] FIGS. 5A-5B are SEM images of an alumina inverse opal film on a silicon substrate, fabricated using AI2O3 nanocrystals, in accordance with certain embodiments.

[0026] FIGS. 5C-5D are SEM images of a zirconia inverse opal film on a silicon substrate, fabricated using Zr0 ₂ nanocrystals, in accordance with certain embodiments.

[0027] FIG. 6 A is an energy dispersive X-ray spectroscopy (EDS) spectra of an alumina inverse opal film, in accordance with certain embodiments.

[0028] FIG. 6B is an EDS spectra of a zirconia inverse opal film, in accordance with certain embodiments.

[0029] FIG. 7A is a TEM image of aluminum oxide (AI2O3) synthesized with an aluminum isopropoxide (AIP):TMAH ratio RAIP/TMAH of about 0.5, in accordance with certain embodiments. Inset (a) of FIG. 7 A shows the crystalline structure of an individual AI2O3 nanocrystal.

[0030] FIG. 7B is a TEM image of zirconium oxide (Zr0 ₂ ) synthesized with a zirconium 1-propoxide (ZIP):TMAH ratio of about 1.0, in accordance with certain embodiments. [0031] FIG. 8 A is a SEM image of a T1O2 inverse opal film, made using nanocrystals synthesized with a TIP:TMAH ratio RTIP/TMAH of about 0.5, in accordance with certain embodiments.

[0032] FIG. 8B is a higher magnification SEM image of the T1O2 inverse opal film of FIG. 17A, in accordance with certain embodiments.

[0033] FIG. 8C is a Raman spectrum of a T1O2 inverse opal film after calcination at 500°C, the Raman spectrum indicating an anatase structure, in accordance with certain embodiments.

[0034] FIG. 8D is an x-ray diffractometry (XRD) spectrum of an anatase T1O2 inverse opal film synthesized with a TIP:TMAH ratio RTIP/TMAH of about 0.5, the TiCh inverse opal film disposed on a silicon substrate after calcination at 500°C, in accordance with certain embodiments.

[0035] FIG. 9A is a SEM image of a T1O2 inverse opal film synthesized with NC, in accordance with certain embodiments.

[0036] FIG. 9B is a SEM image of the T1O2 inverse opal film of FIG. 9A, taken at a lower magnification than in FIG. 9A, in accordance with certain embodiments.

[0037] FIG. 9C is a SEM image of the T1O2 inverse opal film of FIG. 9A, taken at a lower magnification than in FIG. 9A, in accordance with certain embodiments.

[0038] FIG. 10A shows SEM images (with higher magnification on the right) of an AI2O3 inverse opal, synthesized from aluminum isopropoxide - a sol-gel precursor - in accordance with certain embodiments.

[0039] FIG. 10B shows SEM images (with higher magnification on the right) of an AI2O3 inverse opal, synthesized from AI2O3 nanocrystals, the AI2O3 nanocrystals synthesized with an AIP:TMAH ratio RAIP/TMAH of about 0.5, in accordance with certain embodiments.

[0040] FIG. 11 A is a SEM image of a ZrCh inverse opal film, synthesized from nanocrystals synthesized with a ZrlP:TMAH ratio RZITP/TMAH of about 1, in accordance with certain embodiments.

[0041] FIG. 1 IB is a higher magnification SEM image of the ZrC inverse opal film of FIG. 11 A, in accordance with certain embodiments.

[0042] FIG. 12A is a SEM image of a T1O2 inverse opal photonic brick, in accordance with certain embodiments.

[0043] FIG. 12B is a higher magnification SEM image of the T1O2 inverse opal photonic brick of FIG. 12 A, zoomed in on the edge of an individual photonic crystal particle. [0044] FIG. 13 A is a SEM image of a T1O2 inverse opal spherical microparticle, in accordance with certain embodiments.

[0045] FIG. 13B is an optical microscope image of the same microparticles shown in FIG. 13A. Each microparticle is -17 μιη in diameter.

[0046] FIG. 13C is a SEM image of the T1O2 inverse opal spherical microparticle of FIG. 13 A, taken at a higher magnification than in FIG. 13 A, in accordance with certain

embodiments.

[0047] FIG. 13D is a SEM image of the T1O2 inverse opal spherical microparticle of FIG. 13A, taken at a higher magnification than in FIG. 13C, in accordance with certain

embodiments.

[0048] FIG. 14A is a SEM image of a T1O2 inverse opal film incorporating gold nanoparticles (Au NP), in accordance with certain embodiments.

[0049] FIG. 14B is a higher magnification SEM image of the T1O2 inverse opal film incorporating Au P of FIG. 14 A.

[0050] FIG. 15A is a SEM image of spherical T1O2 inverse opal microparticles formed using 420 nanometer (nm) polystyrene (PS) colloids of highly uniform size, as characterized by a polydispersity index (PDI) of 5% , in accordance with certain embodiments.

[0051] FIG. 15B is a SEM image of a fragment of a single photonic particle of the spherical TiCh inverse opal microparticles of FIG. 15 A, in accordance with certain embodiments.

[0052] FIG. 15C is a reflection spectrum acquired from a single photonic particle, where inset (a) shows an optical image of the particles of FIG. 15 A, in accordance with certain embodiments.

[0053] FIG. 16A is a TEM image of a PS colloid modified with 5nm Au NP to form a raspberry-shaped particle, in accordance with certain embodiments.

[0054] FIG. 16B is a TEM image of a PS colloid modified with 7nm platinum (Pt) nanoparticles to form a raspberry-shaped particle, in accordance with certain embodiments.

[0055] FIG. 16C is a TEM image of a PS colloid modified with lOnm palladium (Pd) nanoparticles to form a raspberry-shaped particle, in accordance with certain embodiments.

[0056] FIG. 16D is a SEM image of a T1O2 inverse opal structure with incorporated Au NP, made from the raspberry-shaped particle of FIG. 16 A, in accordance with certain embodiments. [0057] FIG. 16E is a SEM image of a T1O2 inverse opal structure with incorporated Pt nanoparticles, made from the raspberry-shaped particle of FIG. 16B, in accordance with certain embodiments.

[0058] FIG. 16F is a SEM image of a T1O2 inverse opal structure with incorporated Pd nanoparticles, made from the raspberry-shaped particle of FIG. 16C, in accordance with certain embodiments.

[0059] FIG. 17 is a plot of methyl orange concentration (in ppm) versus time, showing catalytic degradation of methyl orange by a T1O2 inverse opal upon exposure to ultraviolet (UV) light, in accordance with certain embodiments.

[0060] FIG. 18A is an optical image of a silica-based inverse opal (IO) paint when dry, in accordance with some embodiments.

[0061] FIG. 18B is an image of the silica-based IO paint of FIG. 18 A, when wet, in accordance with some embodiments.

[0062] FIG. 18C is a plot of the % reflectance for both the dry silica-based IO paint of FIG. 18 A, and for the wet silica-based IO paint of FIG. 18B.

[0063] FIG. 18D is an optical image of a titania-based inverse opal (IO) paint when dry, in accordance with some embodiments.

[0064] FIG. 18E is an image of the titania-based IO paint of FIG. 18D, when wet, in accordance with some embodiments.

[0065] FIG. 18F is a plot of the % reflectance for both the dry titania-based IO paint of FIG. 18D, and for the wet titania-based IO paint of FIG. 18E.

DETAILED DESCRIPTION OF THE INVENTION

[0066] Inverse opals are ordered, porous structures formed from colloidal crystals, and this structuration provides them with many properties. In particular, their porosity facilitates wetting and fluidics studies and applications, and their periodicity facilitates optical and photonic studies and applications. Furthermore, altering their composition can provide inverse opals with additional chemical functionality. Inverse opals are typically comprised of polymers, metals, or metal oxides, and the specific material can be tailored for the application, for example by making colorimetric sensors using stimuli-responsive materials, making porous catalysts using catalytically active materials, or making electrodes using electroactive materials. [0067] The formation of inverse opal films of titania, alumina, zirconia and other non- silica metal oxide compounds has typically been based mostly on a three-step method. First, a sacrificial direct opal template is formed using colloidal particles, such as polymeric colloidal particles. Then, the preformed direct opal structure is infiltrated (or "backfilled") with a metal oxide precursor to form a matrix around the direct opal structure. Transition metal oxide inverse opals have been made previously with a variety of backfilling methods, including dip-coating, dropcasting, spin coating, or vapor-phase deposition. Then, the templating colloidal particles forming the direct opal structure are removed, leaving behind the metal oxide matrix. For example, calcination, which promotes hydrolysis, crystallization, and sintering of the matrix, in addition to removal of the templating polymeric colloids, is commonly used. While major success has been achieved in producing defect-free and crack- free inverse opal silica structures using this methodology, non-silica metal oxide inverse opals, e.g., titania, alumina, zirconia and their mixtures show significant defects, such as cracks, and no methods have been developed so far to overcome this problem.

[0068] Examples of metal oxide precursors used in the conventional method described above include sol-gel precursors (e.g., water-soluble titanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH), as well as highly reactive titanium alkoxides for titania; water- soluble aluminum alkoxides stabilized with acetyl acetone for alumina; and highly reactive zirconium alkoxides that can be stabilized with acetyl acetone for zirconia) and various oxide nanoparticles (both commercially available as well as synthesized precursors).

[0069] The traditional approach outlined above has not, therefore, been suitable for the creation of large-scale crack-free inverse opals of metal oxides. Rather, conventional methods often result in cracked structures due to natural crack formation during the drying of the colloidal crystal film (e.g., via shrinkage), and/or due to a substantial change in the density of the matrix during drying and crystallization (e.g., for crystalline oxides). For example, FIG. 2A illustrates a titanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH) sol-gel precursor and the resulting microstructure (schematic and scanning electron microscope (SEM) image) when the TiBALDH is used to synthesize an inverse opal, in accordance with certain embodiments. FIG. 2B illustrates a crystalline metal oxide (MO) nanoparticle and the resulting microstructure (schematic and scanning electron microscopy (SEM) image) when the MO nanoparticle is used to synthesize an inverse opal, in accordance with certain embodiments. Cracks are clearly visible in the microstructures of FIGS. 2A and 2B. [0070] When backfilling direct opals, these cracks in the opal can undesirably become filled with matrix material. Furthermore, a single infiltration step can be insufficient to fill all the voids within the templating colloidal structure, and therefore is usually repeated via multiple filling/infiltration steps. Multiple filling/infiltration steps can further lead to the formation of non-infiltrated voids within the matrix as well as to the formation of non- templated overlayers. Moreover, many sol-gel precursors have a high reactivity to moisture, and are limited to being processed in inert conditions. In addition, since the conventional approach is based on the pre-existing templating colloidal assemblies, it lacks the flexibility to dynamically control the self-assembly process, making it impossible to achieve certain structures or to incorporate functional components above a certain size.

[0071] The inventors previously developed a co-assembly method using a sol-gel silica precursor (U.S. Patent Application Publication Number 2011/0312080). However, such precursors for other metal oxides lack one or more of the properties needed to produce high quality structures, such as: the ability to prevent destabilization of the colloidal dispersion (e.g., flocculation, formation of aggregates, or inducing disorder), the ability to avoid drying- or calcination-induced cracks, for example by matching the kinetics between the colloidal assembly and hydrolysis/polymerization of the sol-gel precursor, and the ability to avoid excessive densification of the oxide phase during the amorphous-to-crystalline transition of the metal oxide. Moreover, traditional precursors can be incompatible with the incorporation of additional components (e.g. functional nanoparticles, nanocrystals, quantum dots, etc.), under which circumstances the surface chemistry and charges on the precursor are extremely important.

[0072] The present disclosure describes inverse opal fabrication methods that overcome shortcomings of the conventional approaches outlined above, for example by facilitating the formation of highly ordered metal oxide compound opals through a one-step co-assembly procedure using aqueous solutions of specifically designed MO nanocrystals and templating polymeric colloids, leading to high-quality inverse opal structures. For thin films of inverse opals, they can be devoid of lateral cracks above 10,000 repeat units of the templating spheres, which is at least 10 times larger than conventional methods. Methods described herein produce high quality, large area inverse opal films and inverse opal powders, and can be used in combination with microfluidic techniques to form photonic micro-particles (e.g., of a spherical shape). Methods described herein also allow for the incorporation of dopants, such as metal nanoparticles, quantum dots, and various pigments, into the oxide matrix of the inverse opals to achieve color purification and/or saturation, as well as to impart additional functionalities, such as catalytic activity, load and release of guest compounds, or unique light emission capability. In some embodiments, the inverse opal fabrication method does not include the use of a sol-gel precursor.

[0073] In the present disclosure, different co-assembly precursors are employed, and a variety of more widely-applicable methods of making precursors for co-assembly of high- quality inverse opals from a variety of transition metal oxides is set forth. For example, FIG. 2C illustrates a MO nanocrystal with a crystalline core and amorphous phase, prepared according to methods discussed herein, and the resulting microstructure (schematic and scanning electron microscope (SEM) image) when the MO nanocrystal with a crystalline core and amorphous phase is used to synthesize an inverse opal. As can be seen in FIG. 2C, the resulting microstructure of the inverse opal thin film is crack-free for at least about 10,000 repeat units of the photonic crystal structure.

[0074] A number of optical, chemical, and sensing applications are facilitated by defect- free inverted colloidal crystals, and the properties of these inverse opal structures are further expanded by controlling their composition. High-quality, crack-free silica inverse opals with minimal defects can be self-assembled using colloidal crystallization in the presence of a sol- gel precursor, however, this co-assembly process remains challenging for transition metal oxide inverse opals. The present disclosure describes methods for assembling highly ordered, crack-free inverse opals by controlling the state of the matrix precursor, using the synthetic conditions of transition metal oxide nanocrystals to control the surface charge and

crystallinity of the precursor. Some embodiments relate to titania, however the process can be extended to other oxides, such as metal oxides, and other material classes, as well as to other morphologies and templating structures.

Photonic Structures

[0075] Photonic structures that can be produced according to methods described herein include highly ordered inverse opal structures with a variety of morphologies, such as thin films, bricks, balls, and bulk materials. In certain embodiments, these inverse opal structures are crack-free for at least 10,000 repeat units. In certain embodiments, these inverse opal structures are crack-free for at least 1,000 repeat units. In certain embodiments, these inverse opal structures are crack-free for at least 100 repeat units. In certain embodiments, these inverse opal structures are crack-free for at least 5 repeat units. [0076] In certain embodiments, the inverse opal structures are composed of, or substantially of, a metal oxide matrix and air holes.

[0077] Some other exemplary structures include "compound opals" wherein colloidal particles are present as well as the matrix component. Many different types of colloidal particles can be utilized. The colloids can be made from various materials or mixtures of materials. In certain embodiments, the materials are metals, such as gold, palladium, platinum, tin, silver, copper, rhodium, ruthenium, rhenium, titanium, osmium, iridium, iron, cobalt, nickel or combinations thereof. In certain embodiments, the materials are

semiconductor materials, such as silicon, germanium, silicon doped with group III or V elements, germanium doped with group III or V elements, tin doped with group III or V elements, and combinations thereof. In certain embodiments, the materials include catalysts for chemical reactions. In certain embodiments, the materials are oxides, such as silica, titania, zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noble metal oxide, platinum group metal oxide, hafnia, molybdenum oxide, tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides, vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanum oxides, ceria, thorium oxides, uranium oxides, other rare earth oxides, and combinations thereof. In certain embodiments, the materials are metal sulfides, metal chalcogenides, metal nitrides, metal pnictides, and combinations thereof. In certain embodiments, the materials are organometallics, including various metal organic frameworks (MOFs), inorganic polymers (such as silicones), organometallic complexes, and

combinations thereof. In certain embodiments, the colloids are made from organic materials, including polymers, natural materials, and mixtures thereof. In certain embodiments, the material is a polymeric material, such as poly(methyl methacrylate) (PMMA), other polyacrylates, other polyalkylacrylates, substituted polyalkylacrylates, polystyrene (PS), poly(divinylbenzene), poly(vinylalcohol) (PVA), and hydrogels. Other polymers of different architectures can be utilized as well, such as random and block copolymers, branched, star and dendritic polymers, and supramolecular polymers. In certain embodiments, the material is a natural material, such as a protein- or polysaccharide-based material, silk fibroin, chitin, shellac, cellulose, chitosan, alginate, gelatin, and mixtures thereof.

Formation of Inverse Opal Films

[0078] In some implementations described herein, a one-pot procedure is used to synthesize MO nanocrystals in conjunction with colloidal particles to form a photonic structure. FIG. 1 is a diagram of a coassembly method 100 for synthesizing photonic structures, in accordance with certain embodiments. As shown in FIG. 1, a metal complex 102 is combined with an alkyl ammonium compound 104 to form MO nanocrystals 106. The metal complex can be a sol-gel precursor, for example a metal alkoxide such as titanium isopropoxide (TIP). The alkyl ammonium compound 104 can be a quaternary ammonium salt, for example tetramethyl ammonium hydroxide (TMAH). The nanocrystals 106 are then combined with colloidal particles 108 (e.g., polymeric colloidal particles), also referred to herein as templating particles (which can be spherical) to form a suspension 1 10 that includes a solvent (e.g., water). The solvent is evaporated from the suspension 1 10, thereby forming an intermediate product 1 12 that subsequently undergoes calcination to produce an inverse opal 1 14.

[0079] In certain embodiments, the suspension has a final solid content of up to about 20% by weight, for example in a range of about 0.05% to about 10% by weight. In certain embodiments, the suspension has a final solid content of up to 30 w%. In certain

embodiments, the suspension has a final solid content of up to 40 w%. In certain

embodiments, the suspension has a final solid content of up to 50 w%. In certain

embodiments, the suspension has a final solid content of up to 60 w%. In certain

embodiments, the suspension has a final solid content of up to 70 w%. In certain

embodiments, the suspension has a final solid content of up to 80 w%. In certain

embodiments, the suspension has a final solid content of up to 90 w%. In certain

embodiments, the suspension has a final solid content of up to 100 w%.

[0080] In certain embodiments, sizes (e.g., diameters) of the templating particles can range from about 5 nm to several tens or hundreds of microns. Some exemplary sizes include about 100 nm to about 1000 nm to provide specific optical properties and/or improved assembly characteristics that are not largely affected by gravity. In certain embodiments, the size may range from about 100 nm to about 500 nm. As will be apparent to one of skill in the art, many types of sacrificial particles can be utilized.

[0081] In certain embodiments, the colloidal particles can be decorated with

nanoparticles. In certain embodiments, the nanoparticles can include metal (e.g., gold, silver, platinum, palladium, ruthenium, rhodium, cobalt, iron, nickel, osmium, iridium, rhenium, copper, chromium, bimetals, metal alloys, and the like and combinations thereof)

nanoparticles, semiconductor (e.g., silicon, germanium, and the like, pure or doped with elements or compounds of group III or V elements, and combinations thereof) nanoparticles, metal oxide (e.g., silica, titania, zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noble metal oxide, platinum group metal oxide, hafnia, molybdenum oxide, tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides, vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanum oxides, ceria, thorium oxides, uranium oxides, other rare earth oxides, combinations thereof and the like) nanoparticles, metal sulfide nanoparticles, or combinations thereof.

[0082] In certain embodiments, selection of the desired nanoparticles can be based on providing certain desired properties. For example, palladium, platinum, or other noble metal or metal oxide particles can provide catalytic properties, while silver, copper, or oxide (e.g., V2O5) nanoparticles can provide antibacterial properties. Other nanoparticles, such as semiconductor nanoparticles for semiconducting properties, magnetic nanoparticles for magnetic properties, and/or quantum dots for optical properties, can be utilized as desired.

Synthesis of Metal Oxide (MO) Nanocrystals

[0083] Methods of metal oxide (MO) nanocrystal synthesis, according to embodiments described herein, comprise the combination of a metal alkoxide with an onium compound to form a reaction mixture. An onium compound, or onium ion, can be defined as a cation with univalent or multivalent groups covalently bound to a central atom from the pnictogen, chalcogen, or halogen group (e.g., tetramethyl ammonium, phosphonium, etc). The onium compound can include one or more onium cations with polyvalent substitutions (e.g., iminium, imidazolium, or pyridinium), pyrazolium, thiazolium, and/or have the form R ₄ ⁺ (e.g., ammonium, pyrrolidinium), PR ₄ ⁺ (e.g., phosphonium), and SR ₃ ⁺ (e.g., sulfonium), where R can be, for example, H, alkyl, or aryl.

[0084] In certain embodiments, nanocrystals produced according to methods described herein have crystalline cores as well as an amorphous material disposed therein or thereon (e.g., in the form of a coating, layer, shell, and/or the like). The ratio of amorphous material to crystalline material can be controlled by one or more parameters of the synthetic process, such as temperature, stirring speed, reflux conditions, reflux time, pH, identity of base, concentration of ligand, aging time, etc. The biphasic nature of these resulting nanocrystals can have a favorable impact on the inverse opal structures that are subsequently formed from them, perhaps because the amorphous material can accommodate stresses during the assembly process, and thus help to minimize the formation of cracks that are seen in traditional inverse opals produced using traditional synthesis techniques. Metal Oxide (MO) Nanocrystals

[0085] In certain embodiments, the MO nanocrystals have a dimension (e.g., a width or diameter) that is about 1 nm to about 50 nm. In certain embodiments, the MO nanocrystals have a crystalline core and an amorphous phase. In certain embodiments, the degree of crystallinity is estimated from thermal gravimetric analysis (TGA) measurements of the weight percent, for example by subtracting the weight percent at 500°C from the weight percent at 120°C, and then dividing by the weight percent at 500°C.

[0086] FIG. 2D is a plot showing the proportion of amorphous phase to crystalline phase in metal oxide nanocrystals, versus the ratio of TIP/TMAH used to prepare them, in accordance with some embodiments. As shown in FIG. 2D, the degree of crystallinity is in the range of 0.4-0.9, where 0.9 refers to the highest amount of amorphous material.

[0087] In certain embodiments, the MO nanocrystals can be titanium dioxide

nanocrystals, aluminum oxide nanocrystals and/or zirconium oxide nanocrystals.

Incorporation of Functional Particles into Photonic Structures

[0088] The incorporation of a second material component into an inverse opal structure can give rise to synergistic effects, in that it can yield materials with improved or augmented functionalities and properties. For example, the incorporation of metal nanoparticles into inverse opal structures results in the coupling of photonic and plasmonic properties, providing additional control over the optical properties. Furthermore, incorporation of metal nanoparticles may be advantageous for catalysis, greatly expanding the possible applications of these composite materials. Metal nanoparticles have also been used for the antimicrobial, UV-absorbing, sensing, and electrocatalytic properties. Functional particles that can be incorporated into photonic structures described herein include metal (e.g., gold, silver, platinum, palladium, ruthenium, rhodium, cobalt, iron, nickel, osmium, iridium, rhenium, copper, chromium, bimetals, metal alloys, and the like and combinations thereof) nanoparticles, semiconductor (e.g., silicon, germanium, and the like, pure or doped with elements or compounds of group III or V elements, and combinations thereof) nanoparticles, metal oxide (e.g., silica, titania, zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noble metal oxide, platinum group metal oxide, hafnia, molybdenum oxide, tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides, vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanum oxides, ceria, thorium oxides, uranium oxides, other rare earth oxides, and combinations thereof and the like) nanoparticles, metal sulfide nanoparticles, or combinations thereof.

[0089] In certain embodiments, selection of the desired functional particles can be based on providing certain desired properties. For example, Pd or Pt, other noble metal or metal oxide particles can provide catalytic properties, while Ag, copper, or oxide (e.g., V2O5) nanoparticles can provide antibacterial properties. Other nanoparticles, such as

semiconductor nanoparticles for semiconducting properties, magnetic nanoparticles for magnetic properties, and/or quantum dots for optical properties, can be utilized as desired.

[0090] Previous attempts to include metal nanoparticles during co-assembly of titania using sol-gel precursors caused instability of the colloidal solution, resulting in precipitation. By contrast, utilizing the metal oxide nanocrystals described here in conjunction with the metal nanoparticles may allow maintaining the stability of the suspension, allowing formation of inverse opal photonic crystal structure having crack-free structures that extend for 10,000 repeat units of the inverse opal structure.

[0091] In some embodiments, a substrate surface can be activated in order to create hydroxyl groups. The surface activation can be done through exposure of the substrate to high temperature (e.g. calcination in air), and/or to etchants (e.g. piranha solution), and/or plasma.

[0092] In certain embodiments, a substrate can be made from a metal salt or oxide, such as silica, alumina, iron oxide, zinc oxide, tin oxide, alumina silicates, aluminum titanate, beryllia, noble metal oxide, platinum group metal oxide, titania, zirconia, hafnia,

molybdenum oxide, tungsten oxide, rhenium oxide, tantalum oxide, niobium oxide, vanadium oxide, chromium oxide, scandium oxide, yttria, lanthanum oxide, ceria, thorium oxide, uranium oxide, other rare earth oxides, and combinations thereof.

[0093] In other embodiments, the substrate comprises a semiconductor, including at least one of: silicon carbide, silicon, germanium, tin, silicon doped with a group III element, silicon doped with a group V element, germanium doped with a group III element, germanium doped with a group V element, tin doped with a group III element, tin doped with a group V element, and a transition metal oxide.

[0094] In other embodiments, the substrate comprises at least one of a metal and a metal alloy, examples of which include stainless steel, ferritic steel (e.g., an iron-chromium alloy), austenitic steel (a chromium-nickel alloy), copper, nickel, brass, gold, silver, titanium, tungsten, tin, aluminum, palladium, and platinum. [0095] In certain embodiments, the substrate can be made from a ceramic material, such as cordierite, Mullite, zeolite, and natural or synthetic clay.

[0096] In other embodiments, the substrate comprises a combination of composite metal and metal oxide, such as cermet.

[0097] In still other embodiments, the substrate comprises an organic or inorganic material or combination thereof.

[0098] In still other embodiments, the substrate comprises a polymer, such as

polyurethane, and/or comprises at least one of: polyethyleneterephthalate, polystyrene, poly(methyl methacrylate), polyacrylate, polyalkylacrylate, substituted polyalkylacrylate, poly(divinylbenzene), polyvinylpyrrolidone, poly(vinylalcohol), polyacrylamide,

poly(ethylene oxide), polyvinylchloride, polyvinylidene fluoride, polytetrafluoroethylene, other halogenated polymers, hydrogels, organogels, chitin, chitosan, random and block copolymers, branched, star and dendritic polymers, and supramolecular polymers.

[0099] In other embodiments, the substrate can be conductive metal oxide such as indium tin oxide (ITO), fluorine doped tin oxide (FTO) or doped zinc oxide.

[00100] In other embodiments, the substrate can be conductive polymer such as polype- ethyl enedioxythiophene (PEDOT), PEDOT-PSS (polystyrene sulfonate), or a carbon-based conductor (e.g. graphite).

[00101] In still other embodiments, the substrate comprises a natural material, for example including at least one of cellulose, natural rubber (e.g., latex), wool, cotton, silk, linen, hemp, flax, and feather fiber.

Extension of the Procedure to Other Geometries

[00102] In certain embodiments, the photonic crystal structures described herein can be made into numerous different geometries. For instance, the photonic structure can be made into a film, monolith, powder, bricks, spherical particles, shell, coating, cylinder, rod, and other shapes. The structure can have a dimension (e.g., a length, thickness or radius) of from about 1 μπι up to or above 1 cm.

Examples

Example 1: Titania (T1O2) Inverse Opal Structures

[00103] In accordance with certain embodiments, synthetic procedures were designed and optimized specifically to create precursors for the co-assembly of crack-free inverse opals. Previous precursors and attempted precursors either led to unstable suspensions of the assembly solution or cracking of the resulting inverse opal structures (before and/or after the calcination stage). The one-pot procedure described herein synthesizes nanocrystals that, by design, have an amorphous phase intermixed with or adjacent to a crystalline phase, are produced using a minimal number of reagents, and have a desired size and surface charge. Precursors of the present disclosure reduce or eliminate instability of the suspension, and substantially reduce or prevent cracking of the resulting inverse opal structure.

[00104] To prepare T1O2 nanocrystals, a magnetic stirrer and 90-mL of deionized (DI) water were added into a 250 mL flask, and the flask was placed in an ice-bath on top of a magnetic stir plate. A certain amount of TMAH solution (generally 1-4 mL of 25% w/w aqueous TMAH solution) was added and the solution was cooled down to ~2 °C. 1.1 mL (3.7 mmol) of titanium isopropoxide (TIP) was dissolved in 15 mL of 2-propanol and added dropwise (~1 drop/sec) using a dropping funnel to the vigorously stirred aqueous solution of TMAH. After the addition was complete, the reaction mixture was left to stand for -10 minutes in an ice-bath and then for -10 minutes at room temperature. The reaction mixture was then refluxed for 6 hours, cooled, and used without cleaning. This procedure for nanocrystal synthesis is reminiscent of the procedure discussed by Chemseddine and Moritz {Eur. J. Inorg. Chem, 1999, 235-245), in which nanoparticles were characterized for their size, shape, and crystallinity using TEM and XRD, however their amorphous nature was not investigated or mentioned.

[00105] Synthesis of the T1O2 nanocrystals was performed using various molar ratios RTIP/TMAH of TIP/TMAH (RTIP/TMAH), including about 0.3, about 0.5, about 1.00, about 1.4, and about 1.9. FIGS. 3A-3C is a series of transmission electron microscopy (TEM) images of T1O2 nanoparticles for TIP/TMAH ratios RTIP/TMAH of: (A) RTIP/TMAH - 0.3; (B) RTIP/TMAH - 1; (C) RTIP/TMAH - 2. Both the concentration of TMAH and the aging of the particles affect the NC morphology. Depending on the ratio and aging, titania NC are formed with amorphous material. Lower amounts of TMAH and longer aging times can lead to increased crystallinity and less amorphous material.

[00106] To begin fabricating a T1O2 inverse opal film, 100 μΕ of a 5.1 w% colloidal suspension, 250 μΕ of T1O2 nanocrystals (e.g., as prepared according to procedure described above in this section), and 5 mL of deionized water were combined in a glass vial. The final solid content of the colloidal particles was 0.1%. [00107] Opals and inverse opals can be assembled or co-assembled via evaporation- induced self-assembly (EISA). EISA is based on the so-called "coffee ring effect," whereby a droplet of colloidal particles or nanoparticles dries with the particles deposited at the edge of the droplet. In EISA, evaporation currents drive colloidal particles to the air-water interface. Above a certain critical colloid concentration, the meniscus shape causes a thin film to grow on a submerged substrate. EISA has been used extensively to grow direct opals. By adding NC's to the assembly solution, titania-polymer compound opals can form. Depending on the ratio (RTIP/TMAH) and aging time, crack-free titania inverse opals can form.

Example 1(A): Fabrication o/Ti02 Crack-Free Inverse Opal Structures on Silicon Substrates [00108] Silicon (Si) wafer substrates (cut to approximately 1 cm ^χ 5 cm), were cleaned in piranha solution and then vertically suspended in the glass vial containing the colloid/TiCte nanoparticle suspension. The water was evaporated over a period of 1-2 days in a 65°C oven on a pneumatic, vibration-free table to allow the deposition of a compound opal thin film onto the suspended substrate. The compound opal thin films were then calcined at 500°C for 2 hours, with a 5 hour ramp time, to remove the polymer template and to sinter the TiCte nanoparticle, thereby forming an inverse opal film on the Si substrate.

[00109] FIG. 8 A is a SEM image of a T1O2 inverse opal film, made using a T1O2

nanoparticle synthesized with a TIP:TMAH ratio RTIP/TMAH of about 0.5, in accordance with certain embodiments. FIG. 8B is a higher magnification SEM image of the T1O2 inverse opal film of FIG. 8 A.

[00110] FIG. 8C is a Raman spectrum of a T1O2 inverse opal film, produced via calcination at 500°C, the Raman spectrum indicating an anatase structure. FIG. 8D is an X- ray diffraction (XRD) spectrum of a TiCte inverse opal film synthesized with a TIP:TMAH ratio RTIP/TMAH of about 0.5, the TiCte inverse opal film disposed on a silicon substrate after calcination at 500°C, as described above. The XRD spectrum indicates that the inverse opal film has a crystalline structure that is characteristic of anatase TiCte.

[00111] As can be seen in FIGS. 8A-8B, cracks are not visible in the inverse opal film fabricated according to methods of the present disclosure - not even at low magnification. For purposes of comparison, TiCte inverse opals made using nanoparticles that were fabricated using the published synthesis procedure of Chemseddine and Moritz are shown in FIGS. 9A-9C (at varying degrees of magnification). The TiCte inverse opals of FIGS. 9A-9C have minor cracks, and their microstructure ordering is not optimized. Without wishing to be bound by theory, the inventors attribute this fact to the presence of additional charges and molecules in the as-published procedure due to the high pH, which can disrupt the colloidal assembly. By adjusting the synthesis procedure, the inventors have successfully produced high-quality, crack-free inverse opal assemblies for all investigated molar ratios RTIP/TMAH of TIP/TMAH above about 0.3. According to embodiments described herein, after calcination, the T1O2 of the T1O2 inverse opals is present in the anatase phase, according to both Raman spectroscopy (FIG. 8C) and X-ray diffraction (FIG. 8D). In addition to cracks, the size and crystallinity of the nanoparticles will affect some inverse opal properties; for example, smaller particles will lead to more grain boundaries, which can increase catalytic activity.

Example 1(B): Fabrication of T1O2 Inverse Opal Film on Flexible Substrates

[00112] Titania has several attractive inherent properties, such as its high RI,

(photo)catalytic and electrochromic activities. Titania inverse opals films have promising applications in the field of electrochromic (EC) devices. See, e.g., Hua Li, Guillaume

Vienneau, Martin Jones, Balaji Subramanian, Jacques Robichaud and Yahia Djaoued "Crack- free 2D-inverse opal anatase Ti02 films on rigid and flexible transparent conducting substrates: low temperature large area fabrication and electrochromic properties" J. Mater. Chem. C, 2014, 2, 7804. Electrochromism can be defined as the ability of a material to undergo color change induced by an external electric field. Current applications of electrochromism include self-darkening rear view mirrors and electrochromic windows. Ordered titania inverse opal films exhibit improved EC performance due to significantly fast switching times and improved coloration contrasts. Titania inverse opals thus combine optical and catalytic properties of the material and structure, in part because the "slow photon" effect enhances photocatalytic activity due to the photonic nature of the structure. Indeed, titania inverse opals have garnered many reports of their photocatalytic activity, but higher quality structures are still necessary to improve the slow photon effect, as well as to enable better fundamental studies into this slow photon enhancement of photocatalysis. For electrochromic devices, an inverse opal architecture of V2O5, T1O2 and WO3 has recently been reported for its significantly fast switching times and improved coloration contrast. See, e.g., Zhang, J at al. "Energy Dispersive X-ray Spectroscopy Enhanced electrochromic performance of highly ordered, macroporous W03 arrays electrodeposited using polystyrene colloidal crystals as template" Electrochimica Acta, 2013, 99, 1; Li, L. et al. "Improved electrochromic performance in inverse opal vanadium oxide films" J. Mater. Chem., 2010, 20, 7131. [00113] Atomic layer deposition (ALD) is conventionally used for the fabrication of large area, crack-free titania inverse opals films on transparent substrates such as indium tin oxide (ITO) coated glass. The post treatment of opals structures at high temperatures that is typically required in order to crystalize the matrix material and to remove the templating colloids limits the choice of substrates to thermally stable ones. This represents a significant drawback, as the development of wireless technologies and modern electronics requires the design of inexpensive, lightweight, and efficient optoelectronic devices such as portable solar cells or EC devices on flexible substrates. Methodologies described herein facilitate fabrication of large-area defect-free and crystalline titania films on transparent flexible conductive substrates using cost efficient and straightforward fabrication method.

[00114] Large-area crack-free titania (anatase) IO films were fabricated on ITO coated flexible polyethyleneterephthalate (ITO/PET) substrates using the approach described in the current disclosure. The corresponding SEM images are shown in FIGS. 4A-4B. The compound opal films were fabricated through a co-assembly of titania nanocrystals (RTIP/TMAH = 1.35) and PS colloids (420 nm) on a pre-treated ITO/PET substrate. As a pre- treatment, the ITO coated plastic substrates were cleaned using an ultrasonic bath for 3 min, successively in acetone, ethanol and deionized water, dried and plasma treated for 1 min. Following formation of the compound opal film, the PS colloids were removed by dissolution in toluene.

Example 2: Alumina (AI2O3) Inverse Opal Structures

[00115] Synthesis of AI2O3 nanoparticles was performed as follows: Aluminum

isopropoxide (AIP), a sol -gel precursor, was dissolved in 15 mL of 2-propanol and added dropwise to an aqueous solution of TMAH. The reaction mixture was then refluxed for 48 hours. FIG. 7B is a TEM image of AI2O3 nanocrystal synthesized with a TIP:TMAH ratio RAIP/TMAH of about 0.5. The inset (a) of FIG. 7B shows the crystalline structure of an individual AI2O3 nanoparticle.

[00116] Synthesis of AI2O3 inverse opal films was performed using a colloidal dispersion containing 100 μΕ of a 5.1 w% colloidal suspension, 100 μΐ. of AI2O3 nanocrystal solution and 5 mL of deionized water. An inverse opal prepared using a standard alumina precursor (aluminum isopropoxide - a sol-gel precursor) is shown in FIG. 10A for reference, and SEM images of AI2O3 inverse opals assembled from the metal oxide nanocrystal precursors shown in FIG. 7B (with a RAIP/TMAH of about 0.5) are shown in FIG. 10B. Example 3: Zirconia (Zr02) Inverse Opal Structures

[00117] Synthesis of ZrCh nanoparticle was performed as follows: zirconium 1-propoxide (ZIP) solution, a sol-gel precursor, was dissolved in 15 mL anhydrous 2-propanol in a glovebox. The solution was subsequently removed and added dropwise to an aqueous solution of TMAH, followed by a 12 hour reflux. FIG. 7C is a TEM image of ZrCh nanoparticle synthesized with a ZIP:TMAH ratio RZIP/TMAH of about 1.0.

[00118] Synthesis of ZrCh inverse opal films was performed using a colloidal dispersion containing 100 μΙ_, of a 5.1 w% colloidal suspension, 400 μΙ_, of ZrCh nanoparticle and 4.5 mL of deionized water. FIGS. 11 A-l IB are SEM images (at varying magnifications) of a ZrCh inverse opal film, synthesized as outlined above, where the ZrCh nanoparticle were synthesized with a ZIP:TMAH ratio RZIP/TMAH of about 1.0 (i.e., the nanoparticle precursor shown in FIG. 7C).

Example 4: Incorporation of Gold Nanoparticle s into Titania Inverse Opals through 3-phase Co-Assembly.

[00119] Gold nanoparticles (Au NP) were incorporated into inverse opal films (shown in FIGS. 14A and further magnified in FIG. 14B), bricks, and balls (shown in FIG. 15A and further magnified in FIG. 15B) by adding a polyethylene glycol (PEG)-modified Au NP solution (to a final weight percent of 0.1 to 5 w%) into a co-assembly precursor solution comprising an aqueous solution of polymer (420 nm polystyrene (PS)) colloids with a polydispersity index (PDI) of <5% and T1O2 NCs. Water was added to achieve the desired overall precursor concentrations described above for films, bricks, and photonic balls.

Incorporation of Au NP intensifies the red appearance (shown as the lighter central regions of the nanoparticles shown in the grayscale image of FIG. 15C) of the photonic structure through selective absorption of undesired reflections. Exemplary effects on the optical spectra are shown for photonic balls in the reflection spectrum, acquired from a single photonic particle, of FIG. 15C. The photonic peak at -680 nm is enhanced by the Au NP absorption (-550 nm). Inset (a) shows an optical image of the particles of FIG. 15A.

Example 5: Incorporation of gold, platinum, and palladium nanoparticles into titania inverse opals through 2-phase co-assembly using raspberry particles. [00120] The inventors have previously reported on decorated particles (WO 2014/210608). Decorated particles, or "raspberry-shaped particles," are composite metal-polymer colloidal particles comprising metal nanoparticles that are covalently bound to a chemically modified surface of polystyrene (PS) colloids, with homogeneous metal nanocrystals distribution at the pore surface. FIGS. 16A-16C are TEM images of raspberry particles formed with a PS colloid modified with: 5nm Au nanocrystals, 7nm platinum (Pt) nanoparticles, and lOnm palladium (Pd) nanoparticles, respectively. These composite PS - metal nanoparticles colloids can be employed as template particles in the assembly of inverse opal scaffolds. Inverse opals can be assembled using a protocol similar to the co-assembly of TiCh inverse opals described herein, but with raspberry-shaped particles substituted for the

unfunctionalized PS colloids. A colloidal suspension of raspberry-shaped particles was mixed with 250 μΙ_, of T1O2 nanocrystal solution and deionized water to achieve a 0.1% final solid content of the colloidal suspension. Si wafer substrates (cut to approximately 1 cm ^χ 5 cm), cleaned in piranha solution, were vertically suspended in a vial containing the raspberry particles/TiC nanoparticle suspension. The water was evaporated over a period of 1-2 days in a 65°C oven on a pneumatic vibration-free table, to allow the deposition of a thin film onto the suspended substrate. The compound opal films were then calcined at 500°C for 2 hours, with a 5 hour ramp time, to remove the polymer template and to sinter the T1O2 NC. SEM images of exemplary inverse opal structures with incorporated metal nanoparticles are shown in FIGS. 16D-16F (where the metal nanoparticles is Au, Pt and Pd, respectively).

[00121] In some embodiments, the metal nanoparticle incorporation method described above yields composite inverse opals with metal nanoparticles found exclusively at the air/metal oxide (e.g., T1O2) interface of the inverse opal matrix. In such configurations, each metal nanoparticle is accessible/exposed to chemical reagents or analytes that are introduced into the scaffold via liquid or gas phase, which is advantageous for applications such as catalysis and surface-enhanced Raman scattering (SERS). In addition, the confinement of plasmonic nanoparticles (e.g., Au NP) to the surface of the inverse opal scaffold induces a controlled spectral modification of the inverse opal reflection, making this material a versatile platform for photonic/plasmonic colorimetric sensing and effect pigmentation with angle- independent coloration resulting from the metal nanoparticles absorption and iridescence imparted by the inverse opal's nanoscale periodicity.

Example 6: Photonic Crystal Bricks [00122] Assembly of T1O2 inverse opal photonic bricks, or "freeform" photonic bricks were grown on the inner walls of 20 mL glass vials containing co-assembly solution. The thickness of the photonic bricks was tunable by adjusting the concentration of the precursor solution. For example, in one implementation, 300 μΙ_, of a 5.1 w% colloidal suspension, 750 μΐ, of T1O2 nanoparticles and 5 mL of deionized water were combined in a glass vial. The final solid content of the colloidal suspension was 0.3%. The solvent content of the colloidal suspension was evaporated over a period of 1-2 days in a 65 °C oven on a pneumatic vibration-free table, to allow the deposition of a film onto the inner walls of the vial. In this colloidal concentration regime, the formation of photonic bricks occurs spontaneously due to natural cracking of the film deposited on the wall, thereby facilitating particulate release for collection during calcination (e.g., at 500 °C) and sintering. An SEM image of "freeform" T1O2 opal photonic bricks is shown in FIG. 12A, and a higher magnification SEM image of the T1O2 inverse opal photonic brick of FIG. 12 A, zoomed in on the edge of an individual photonic crystal particle.

Example 7: Photonic Crystal Balls

[00123] The formation of spherical inverse opal particles, or "photonic balls," in accordance with the present disclosure was performed as follows: An aqueous solution containing -0.5 wt-% of polystyrene colloids (256 nm, PDI~5%) and -0.7 wt-% of T1O2 nanoparticles was emulsified using a t-junction microfluidic device with 150 μπι channel width. The continuous phase contained 0.5 wt-% of a surfactant in Novec 7500 (3M). Upon evaporation of water from the droplets, the resulting spherical microparticles were calcined at 600°C for two hours. SEM images of the resulting T1O2 inverse opal spherical microparticles are shown in FIGS. 13A and 13C-13D (at varying magnification), and an optical microscope image of the TiCh inverse opal spherical microparticles is shown in FIG. 13B. The T1O2 inverse opal spherical microparticles have an iridescent appearance that is dominated by a blue color, when white light is reflected in a specular configuration. Further details concerning photonic balls can be found in U.S. Patent Application Publication Number 2014/0254017.

[00124] In the foregoing examples, the final solid content of the suspension prior to calcining is typically about 1 - 10 w% initially for photonic balls, about 0.1 w% for films, and about 0.3 w% for shards. In each case, the water or other solvent component of the suspension evaporates over time, and therefore the solid content can increase over time. Applications

[00125] Inverse opals made according to certain embodiments herein can enable and/or enhance a variety of applications. For example, in catalysis, T1O2 is known to oxidize organic molecules upon irradiation with UV light. These T1O2 inverse opals were tested for their photocatalytic activity using methyl orange as a test molecule. The concentration of methyl orange in solution was monitored with UV-vis spectroscopy. FIG. 17 is a plot of methyl orange concentration (in ppm) versus time, showing catalytic degradation of methyl orange by a T1O2 inverse opal upon exposure to ultraviolet (UV) light.

[00126] T1O2, AI2O3, ZrCh, and other metal oxide inverse opal films, powders and microparticles described herein combine the benefits of the inverse opal structure (including both periodicity and porosity) with the chemical and physical properties of the oxides and their mixtures (including refractive index, absorption properties, catalytic activity, electroactivity, or strength). Metal oxide inverse opals of the present disclosure can be useful for applications such as:

• structural color dyes in paint formulations (with the ability to retain color in

solutions)

• structural color dyes in cosmetic products, with additional functionality such as antimicrobial or sunscreen properties

• heterogeneous / dispersive catalytic substrates or supports

• solar cells and fuel cells (with the metal oxide as the active or support medium)

• other electrode uses, such as photoelectrochemical cells, capacitors, and batteries

• sensing substrates, for example in colorimetric indicators or for Raman

enhancement

• drug loading and release vehicles

• filters and membranes, for separation or air filtration

• photonic applications, such as lasing and waveguiding

• other optical applications, such as smart displays or other electrochromic materials

• substrates for microelectronics applications

• coatings for cooling devices [00127] For example, the catalytic activity of titania inverse opals is demonstrated in FIG 17. Titania inverse opal particles in accordance with some embodiments is shown to catalyze the degradation of methyl orange under UV light.

[00128] As another example, the color retention performance of titania inverse opal -based paint is compared with that of silica inverse opal paint in FIGS. 18A-18F. FIG. 18A is an optical image of a silica-based inverse opal (IO) paint when dry, in accordance with some embodiments. FIG. 18B is an image of the silica-based IO paint of FIG. 18 A, when wet, in accordance with some embodiments. FIG. 18C is a plot of the % reflectance for both the dry silica-based IO paint of FIG. 18 A, and for the wet silica-based IO paint of FIG. 18B.

[00129] FIG. 18D is an optical image of a titania-based inverse opal (IO) paint when dry, in accordance with some embodiments. FIG. 18E is an image of the titania-based IO paint of FIG. 18D, when wet, in accordance with some embodiments. FIG. 18F is a plot of the % reflectance for both the dry titania-based IO paint of FIG. 18D, and for the wet titania-based IO paint of FIG. 18E.

[00130] Embodiments described herein, comprising the formation of high-quality structures through coassembly within droplets, can be used for direct printing of T1O2 photonic structures using inkjet printing technology.

[00131] As used herein, the terms "about" and "approximately" generally mean plus or minus 10% of the value stated, e.g., a value of about 250 would include 225 to 275, and about 1,000 would include 900 to 1, 100.

[00132] Upon review of the description and embodiments provided herein, those skilled in the art will understand that modifications and equivalent substitutions may be performed in carrying out the invention without departing from the essence of the invention. Thus, the invention is not meant to be limiting by the embodiments described explicitly above.