PREPARATION OF S1O2-T1O2 COMPOSITE AEROGELS AND Si02@TiC>2 CORE-SHELL AEROGELS WITH HIGH

THERMAL STABILITY AND ENHANCED PHOTOCATALYSIS

[0001] The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence

Livermore National Laboratory.

FIELD OF THE INVENTION

[0002] The present invention relates to aerogels, and more particularly, this invention relates to thermally stable Si02-Ti0 ₂ composite and Si0 ₂ @Ti0 ₂ core-shell aerogel photocatalysts.

BACKGROUND

[0003] T1O2 is a metal oxide semiconductor that is of interest for photocatalysis and other uses. However, efforts to maximize photocatalytic activity, in particular high surface area, good crystallinity and controlled crystalline phase composition, have heretofore been difficult to achieve.

[0004] Aerogels are a class of nanoporous materials prepared by the controlled drying of wet gels formed by sol-gel derived inorganic or organic crosslinked networks. Typically, supercritical drying is employed. Controlled drying avoids shrinkage and pore collapse, preserving the fine pore network structure in the dried material, and thus resulting in a highly porous, low-density solid.

[0005] Owing to their unique textural and structural properties, T1O2 aerogels are interesting choices for development of high performance photocatalysts. Preparation of titania aerogels is most often carried out by the controlled hydrolysis and

polycondensation of titanium alkoxides precursors in alcohol solvents, yielding monolithic alcogels that are converted into aerogels after subsequent supercritical drying. However, titanium alkoxide precursors exhibit some disadvantages including high cost, lower commercial availability, difficult storage, and necessity of precise control of synthetic parameters to obtain good quality monolithic gels.

[0006] Moreover, a significant drawback of existing titania nanomaterials is their lack of thermal stability, which leads to large decrease of surface area and porosity, particle sintering and anatase-to-rutile phase transformation upon annealing. Such behavior limits the use of titania aerogels for high temperature applications and affects its use in photocatalysis as well, because post-synthesis thermal treatments are often required to crystallize the amorphous sol-gel derived titania (preferably into anatase structure) and or/to reduce the number of crystal lattice defects, which presence is detrimental to the material photoactivity. Furthermore, as calcination above 600°C in air causes titania dioxide to undergo anatase-to-rutile phase transformation, formation of anatase-based nanomaterial with high thermal stability (e.g., up to 1000°C) has proven challenging.

SUMMARY

[0007] According to one inventive concept, a method for forming an aerogel includes forming a S1O2 gel, forming a mixture of the S1O2 gel and a TiCU-derived precursor sol, wherein the TiCU sol is comprised of TiCU and a solvent, forming a SiC /Ti wet gel, drying the SiC /Ti wet gel, and heating the dried SiC /TiC aerogel.

[0008] According to another inventive concept, a product includes a SiC -TiC composite aerogel having a plurality of S1O2 and T1O2 particles.

[0009] According to yet another inventive concept, a product includes a Si02@Ti02 core-shell aerogel having a plurality of Si02@Ti02 particles, where a core of each of the particles comprises S1O2 and a shell of each of the particles comprises T1O2.

[0010] Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1A is flowchart of a method, according to one inventive concept.

[0012] FIG. IB is flowchart of a method, according to one inventive concept.

[0013] FIG. 1C is flowchart of a method, according to one inventive concept.

[0014] FIG. 2 is a series of schematic drawings of sol-gel synthesis of Si0 ₂ /Ti02 aerogels, according to various inventive concepts.

[0015] FIG. 3A is an image of Si0 ₂ @Ti0 ₂ core-shell wet gels of increasing Ti0 ₂ concentrations, according to one inventive concept.

[0016] FIGS. 3B-3D are images of Si0 ₂ @Ti0 ₂ core-shell aerogels, according to one inventive concept.

[0017] FIG. 3E is an image of a Si0 ₂ -Ti0 ₂ composite wet gel, according to one inventive concept.

[0018] FIG. 3F is an image of a Si0 ₂ -Ti0 ₂ composite aerogel, according to one inventive concept.

[0019] FIG. 4A is a plot of N ₂ physisorption of Si0 ₂ @Ti0 ₂ core-shell aerogel (TID route) annealed at varying temperatures, according to one inventive concept.

[0020] FIG. 4B is a plot of N ₂ physisorption of Si0 ₂ -Ti0 ₂ composite aerogel (EAG route) annealed at varying temperatures, according to one inventive concept. [0021] FIG. 5A part (a) is a plot of specific surface area in absolute values for samples of Si02@Ti0 ₂ core-shell aerogel (TID route) after thermal annealing at various temperatures, according to one inventive concept.

[0022] FIG. 5A part (b) is a plot of specific surface area as normalized percentage values for samples of Si0 ₂ @Ti0 ₂ core-shell aerogel (TID route) after thermal annealing at various temperatures, according to one inventive concept.

[0023] FIG. 5B part (a) is a plot of specific surface area in absolute values for samples of Si0 ₂ -Ti0 ₂ composite aerogel (EAG route) after thermal annealing at various temperatures, according to one inventive concept.

[0024] FIG. 5B part (b) is a plot of specific surface area as normalized percentage values for samples of Si0 ₂ -Ti0 ₂ composite aerogel (EAG route) after thermal annealing at various temperatures, according to one inventive concept.

[0025] FIG. 5C part (a) is a plot of specific surface area in absolute values for samples of bare Ti0 ₂ aerogel after thermal annealing at various temperatures, according to one inventive concept.

[0026] FIG. 5C part (b) is a plot of specific surface area as normalized percentage values for samples of bare Ti0 ₂ aerogel after thermal annealing at various temperatures, according to one inventive concept.

[0027] Fig. 6A is a plot of powder X-ray diffractograms of Si0 ₂ @Ti0 ₂ core-shell aerogels (TID route) annealed at 600°C, 800°C, and 1000°C, according to one inventive concept. [0028] Fig. 6B is a plot of powder X-ray diffractograms of Si02-Ti0 ₂ composite aerogels (EAG route) annealed at 600°C, 800°C, and 1000°C, according to one inventive concept.

[0029] Fig. 6C is a plot of powder X-ray diffractograms of Ti0 ₂ aerogels annealed at 600°C, 800°C, and 1000°C, according to one inventive concept.

[0030] FIG. 6D part (a) is a schematic drawing of the crystallite growth of

Si0 ₂ @Ti0 ₂ core- shell aerogel (TID route) after heat treatment, according to one inventive concept.

[0031] FIG. 6D part (b) is a schematic drawing of the crystallite growth of Si0 ₂ -Ti0 ₂ composite aerogel (EAG route), according to one inventive concept.

[0032] FIG. 6D part (c) is a schematic drawing of the crystallite growth of Ti0 ₂ aerogel, according to one inventive concept.

[0033] FIG. 7 A is a plot of Rhodamine B dye photodegradation.

[0034] FIG. 7B is a plot of photocatalytic degradation of Rhodamine B dye in the presence of Si0 ₂ @Ti0 ₂ aerogel particles, according to one inventive concept.

[0035] FIG. 7C is a plot of photocatalytic degradation of Rhodamine B dye in the presence of Si0 ₂ -Ti0 ₂ composite aerogel particles, according to one inventive concept.

[0036] FIG. 7D is a plot of photocatalytic degradation of Rhodamine B dye in the presence of Ti0 ₂ aerogel particles, according to one inventive concept.

[0037] FIG. 7E is a bar graph depicting thermal treatment temperature effect on RhB photodegradation first-order kinetic constants (Kobs) for a Si0 ₂ -Ti0 ₂ composite aerogel, a Si0 ₂ @Ti0 ₂ core-shell aerogel, and a bare Ti0 ₂ aerogels. [0038] FIG. 8 is a comparison of photocatalytic activity in terms of on Crystal Violet dye photodegradation first-order kinetic constants (Kobs)for Si02-Ti0 ₂ composite aerogels and Si0 ₂ @Ti0 ₂ core-shell aerogel treated at 1000°C and high performance commercial photocatalyst P25 (non treated and treated at 1000°C), according to one inventive concept.

DETAILED DESCRIPTION

[0039] The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

[0040] Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

[0041] It must also be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless otherwise specified.

[0042] As also used herein, the term "about" when combined with a value refers to plus and minus 10% of the reference value. For example, a length of about 1 μιη refers to a length of 1 μιη + 0.1 μιη.

[0043] It is also noted that, as used in the specification and the appended claims, wt% is defined as the percentage of weight of a particular component is to the total weight of the mixture. Moreover, mol% is defined as the percentage of moles of a particular component is to the total moles of the mixture. [0044] In one general inventive concept, a method for forming an aerogel includes forming a S1O2 gel, forming a mixture of the S1O2 gel and a TiCU-derived precursor sol, wherein the TiCU sol is comprised of TiCU and a solvent, forming a SiC /Ti wet gel, drying the SiC /Ti wet gel, and heating the dried SiC /TiC aerogel.

[0045] In another general inventive concept, a product includes a Si -Ti composite aerogel having a plurality of S1O2 and T1O2 particles.

[0046] In yet another general inventive concept, a product includes a Si02@Ti02 core-shell aerogel having a plurality of Si02@Ti02 particles, where a core of each of the particles comprises S1O2 and a shell of each of the particles comprises T1O2.

[0047] A list of acronyms used in the description is provided below.

SBET Specific surface area (Brunauer, Emmett and Teller)

C Celsius

C0 ₂ carbon dioxide

CV Crystal violet

DMF dimethylformamide

EAG epoxide-assisted gelation

h hour(s)

H ₂ 0 water

RhB Rhodamine B

S1O2 silicon dioxide, silica

TID thermo-induced deposition

TiCU titanium tetrachloride

Ti0 ₂ titanium dioxide, titania XRF X ray Fluorescence spectroscopy

[0048] Owing to their highly porous nature and unique structural properties, T1O2 aerogels prepared by sol-gel methodology and supercritical drying are of interest for development of high performance photocatalysts for environmental remediation and solar energy conversion processes. Nevertheless, controlled formation of nanocrystalline T1O2 aerogels has heretofore proven challenging, as the required crystallization by post- synthesis high temperature thermal treatments may lead to uncontrolled crystallite growth and sintering, as well as irreversible anatase-to-rutile phase transformation, structural changes which are highly detrimental to photocatalytic performance.

[0049] To address such drawbacks, various inventive concepts described herein include processes for fabrication of Si -Ti composite aerogels and Si02@ i02 core- shell aerogels using TiCU precursor. For example, described herein are two novel TiCU- based non-alkoxide sol-gel routes for the synthesis of SiC /TiC nanocomposite aerogels. In the first route, SiC -TiC composite aerogels are obtained by epoxide-assisted gelation (EAG) of TiCU/solvent solution in the presence of S1O2 aerogel particles. In the second route, a TiCU/solvent solution may be used to prepare Si02@Ti02 core-shell aerogels by a facile one-step thermo-induced deposition (TID) of T1O2 on silica wet gel supports. After controlled drying, e.g., in supercritical CO2, high surface area silica-titania aerogels are obtained as fragile monoliths or fine powders (EAG route) or as crack-free monoliths (TID route).

[0050] Various inventive concepts described herein may be used to fabricate silica- titania (SiC /TiC ) aerogels that display excellent structural and textural properties including high surface area, large pore volume, and outstanding thermal stability upon high temperature annealing. Importantly, the materials have greater thermal stability than T1O2 aerogels, and in some approaches, demonstrate anatase nanocrystals (9-15 nm), robust mesoporous structure, and high surface area even after thermal treatment at 1000°C. Such improved structural properties resulted in further improvement of photocatalytic activity of Si /TiC aerogels after high temperature annealing (to as high as 1000°C). Without wishing to be bound to any theory, it is believed that the high photocatalytic performance of the Si0 ₂ /Ti02 aerogels formed by methods described herein may be due to the better photocatalytic activity of the anatase phase of T1O2 compared to other T1O2 crystalline phases, as well as excellent structural and textural properties of silica-titania aerogels, which contrast significantly with those of unsupported T1O2 aerogels.

[0051] FIG. 1A shows a method 100 for forming a SiC /Ti aerogel, in accordance with one embodiment. As an option, the present method 100 may be implemented to construct structures such as those shown in the other FIGS, described herein. Of course, however, this method 100 and others presented herein may be used to form structures for a wide variety of devices and/or purposes which may or may not be related to the illustrative embodiments listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 1A may be included in method 100, according to various embodiments. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with the various methods.

[0052] In one inventive concept described herein, a method 100 for forming an aerogel includes operation 102 of forming a S1O2 gel. [0053] Operation 104 of method 100 includes forming a mixture of the S1O2 gel and a TiCU-derived precursor sol. The TiCU-derived precursor sol includes TiCU and a solvent. In some approaches, the solvent may include a mixture of dimethylformamide and water. The TiCU sol includes TiCU and a solvent. This and other approaches described herein may employ solvents used in conventional sol-gel processes, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), etc. A preferred solvent for this step is DMF with a small amount of water. The concentration of TiCU may vary but H ₂ 0:TiCL4 ratio is preferably maintained at a ratio of 6: 1. The dispersion may be agitated for a period of time, e.g., greater than 6 h. In a preferred approach, the dispersion may be subjected to magnetic stirring for about 24 h. The operation 104 of forming of the TiCU sol may be used in this and other embodiments described herein

[0054] Operation 106 includes forming a Si0 ₂ /Ti0 ₂ wet gel.

[0055] Next, the gel may be washed with a solvent. In a preferred approach, the gel is washed with DMF, ethanol, and acetone. Again, however, solvents used for washing in conventional sol-gel processes may be used in the washing step of this and other approaches described herein.

[0056] Operation 108 includes drying the Si0 ₂ /Ti0 ₂ wet gel. After washing, supercritical drying may be performed on the gel to convert the wet gel into an aerogel. In some approaches, the drying may include supercritical carbon dioxide drying.

Supercritical drying techniques and materials used in conventional sol-gel processing may be used in the drying step in this and other approaches described herein. In one approach, the gel is dried in supercritical carbon dioxide (C0 ₂ ). Supercritical drying using other supercritical fluids (e.g., ethanol, isopropanol, etc.) may be performed in other approaches.

[0057] Operation 110 includes heating the dried Si0 ₂ /Ti02 aerogel for crystallizing titania in photocatalytic active crystalline phases (preferably anatase nanocrystals).

[0058] After supercritical drying, a calcining step may be performed. In some approaches, the heating may be at a temperature in a range of about 600°C to about 1000°C under ambient air atmosphere.

[0059] FIG. IB shows a specific method 120 for forming a Si0 ₂ -Ti0 ₂ composite aerogel by epoxide-assisted synthesis, in accordance with one embodiment. As an option, the present method 120 may be implemented to construct structures such as those shown in the other FIGS, described herein. Of course, however, this method 120 and others presented herein may be used to form structures for a wide variety of devices and/or purposes which may or may not be related to the illustrative embodiments listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. IB may be included in method 120, according to various embodiments. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with the various methods.

[0060] In one inventive concept described herein, a method 120 for forming Si0 ₂ - Ti0 ₂ composite aerogels includes an epoxide-assisted gelation (EAG), in which Si0 ₂ dried aerogel powder (particles) may be dispersed in TiCU sol.

[0061] According to method 120, the operations of forming a Si0 ₂ gel result in forming a powder of Si0 ₂ aerogel particles. [0062] Operation 122 includes forming a S1O2 wet gel.

[0063] Operation 124 includes drying the Si0 ₂ wet gel by supercritical drying method to form a Si0 ₂ aerogel. Processes of drying the Si0 ₂ wet gel are described herein for other approaches.

[0064] Operation 126 includes crushing the Si0 ₂ aerogel into a powder comprising Si0 ₂ aerogel particles. Methods for crushing the Si0 ₂ aerogel into a powder may include grinding with mortar and pestle, bath sonication, tip sonication, ball milling, orbital mixer, etc.

[0065] Operation 128 of method 120 includes forming a mixture of the powder of Si0 ₂ aerogel particles dispersed in TiCU sol. The TiCU sol includes TiCU and a solvent, as described in inventive concepts herein.

[0066] Next, gelification (also referred to herein as gelation) is induced by addition of an epoxide. Operation 130 of method 120 includes forming the Si0 ₂ /Ti0 ₂ wet gel comprises adding epoxide to a mixture of Si0 ₂ aerogel particles dispersed in the TiCU sol for gelation of the mixture.

[0067] A known epoxide may be used, e.g., an epoxide used in conventional sol-gel processes. Illustrative epoxides include, but are not limited to, propylene oxide, ethylene oxide, trimethylene oxide, dimethylene oxide, epichlorohydrin, etc. In a preferred approach, gelification is induced by a mixture of DMF and propylene oxide.

[0068] As described in method 100, drying techniques for forming an Si0 ₂ /Ti0 ₂ aerogel may be applied to method 120 of forming a Si0 ₂ -Ti0 ₂ composite aerogel.

Operation 132 of method 120 includes drying the Si0 ₂ -Ti0 ₂ wet gel following methods described herein. In some approaches of the EAG process for forming Si0 ₂ -Ti0 ₂ composite aerogels, a controlled supercritical drying results in high surface area silica- titania aerogels in the form of fragile monoliths or fine powders.

[0069] Moreover, as described for method 100 of forming a Si0 ₂ /Ti02 aerogel, heating the dried the Si0 ₂ -Ti0 ₂ composite aerogel of operation 134 of method 120 follows similar methods described herein.

[0070] In some approaches, the Si0 ₂ -Ti0 ₂ composite aerogel has plurality of silica (e.g. Si0 ₂ ) and titania (e.g. Ti0 ₂ ) particles in the aerogel. In some approaches, the Si0 ₂ - Ti0 ₂ composite aerogels formed by an EAG process may have more titania than silica on weight basis; however relative amounts may vary. In some approaches, the chemical composition of Si0 ₂ -Ti0 ₂ composite aerogels (content of Si0 ₂ and Ti0 ₂ ) may be tuned by adjusting the added amount of silica aerogel particles during the step of forming a mixture of Si0 ₂ aerogel particles dispersed in the TiCU sol (operation 128 of method 120). Using such approach, Si0 ₂ -Ti0 ₂ composite aerogels may have Ti0 ₂ content in a range of 50% to 90% in weight, as determined by X-ray Fluorescence spectroscopy (XRF). In some approaches, adding more Si0 ₂ particles to increase Si0 ₂ content lowers the content of Ti0 ₂ (e.g. less than 50% weight of Ti0 ₂ ); however, the resulting gels may likely be too fragile for further processing.

[0071] In some approaches, the Si0 ₂ -Ti0 ₂ composite aerogel includes

nanocrystallites of pure anatase phase Ti0 ₂ . In some approaches, an average crystalline size of anatase phase nanocrystallites in the shells may be less than 15 nanometers.

[0072] FIG. 1C shows a method 150 for forming a Si0 ₂ @Ti0 ₂ core-shell aerogel by a newly developed thermo-induced deposition (TID) method, in accordance with one embodiment. As an option, the present method 150 may be implemented to construct structures such as those shown in the other FIGS, described herein. Of course, however, this method 150 and others presented herein may be used to form structures for a wide variety of devices and/or purposes which may or may not be related to the illustrative embodiments listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 1C may be included in method 150, according to various embodiments. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with the various methods.

[0073] According to another inventive concept, a method 150 includes forming a composite Si02@Ti0 ₂ core-shell aerogel in one step by thermo-induced deposition (TID). Method 150 begins with operation 152 of forming a Si0 ₂ gel. In various approaches, the Si0 ₂ gel may be a Si0 ₂ wet gel.

[0074] According to method 150, the Ti0 ₂ may be deposited on the Si0 ₂ in one step using TiCU as a precursor during TID. Operation 154 includes soaking the Si0 ₂ wet gel in a bath comprising TiCU sol. The bath may be heated for a duration of time. The Si0 ₂ wet gel may be modified by soaking the Si0 ₂ wet gel in TiCU sol at a moderate temperature. Increased temperature is used to promote precipitation of Ti0 ₂ induced by thermohydrolysis of TiCU sol. For example, in preferred approaches, the soaking occurs at various temperatures, for example, initial soaking is at ambient temperature for about 24 hours, and then the temperature is raised so the soaking continues at an elevated temperature, e.g., above 65°C, preferably about 80°C. [0075] The TiCU sol includes TiCU and a solvent. This and other approaches described herein may employ solvents used in conventional sol-gel processes. A preferred solvent for this step is DMF with a small amount of water.

[0076] Titania deposition on the S1O2 wet gel involves soaking the Si0 ₂ wet gel in TiCU sol for a period of time at ambient temperature. In some approaches Si0 ₂ wet gel is soaked at ambient temperature for greater than 6 hours. In preferred approaches, Si0 ₂ wet gel is soaked at ambient temperature for about 24 hours. Then the temperature is elevated from ambient temperature and the soaking of Si0 ₂ wet gel in TiCU sol continues at a temperature above 65°C, preferably about 80°C for a duration of greater than 6 h. In a preferred approach, Si0 ₂ wet gel continues soaking in TiCU sol at 80°C for about 24 h.

[0077] Next, the Si0 ₂ @Ti0 ₂ core-shell wet gel may be washed with a solvent. In a preferred approach, the gel is washed with DMF, ethanol and acetone. Again, however, solvents used for washing in conventional sol-gel processes may be used in the washing step of this and other approaches described herein.

[0078] After washing, operation 156 includes supercritical drying of the Si0 ₂ @Ti0 ₂ core-shell gel. Supercritical drying techniques and materials used in conventional sol-gel processing may be used in the drying step in this and other embodiments described herein. In one approach, the gel is dried in supercritical C0 ₂ . Supercritical drying using other supercritical fluids (e.g., ethanol, isopropanol, etc) may be also used in other approaches.

[0079] After supercritical drying, operation 158 includes a calcining step, preferably heating the dried Si0 ₂ @Ti0 ₂ core-shell gel at temperatures above 1000°C. Similar performance of the resulting structures is expected at temperatures reasonably above 1000°C, e.g., up to 1250°C.

[0080] In some approaches of the TID process for forming Si02@Ti0 ₂ core-shell aerogels, a controlled supercritical drying may result in high surface area silica-titania aerogels in the form of crack-free monolithic structures.

[0081] According to method 150, the formed aerogel is a Si0 ₂ @Ti0 ₂ core-shell aerogel. In some approaches, the Si0 ₂ @Ti0 ₂ core-shell aerogels have a plurality of Si0 ₂ particles coated with Ti0 ₂ . The cores are Si0 ₂ particles and the corresponding shells are primarily or entirely Ti0 ₂ . In some approaches, the Si0 ₂ @Ti0 ₂ core-shell aerogels formed by a TID process may have more silica to titania on a weight basis; however relative amounts may vary. In some approaches, the chemical composition of Si0 ₂ @Ti0 ₂ core-shell aerogels (content of Si0 ₂ and Ti0 ₂ ) may be tuned by adjusting the TiCU concentration in precursor sol employed (operation 154 for method 150). Using such approach, it is possible to obtain Si0 ₂ @Ti0 ₂ core-shell aerogels may have Ti0 ₂ content in a range of 20% to 65% in weight, as determined by XRF analysis.

[0082] In some approaches, the Si0 ₂ @Ti0 ₂ core-shell aerogel includes particles where the shells of the particles may include pure anatase phase Ti0 ₂ . In some approaches, an average crystalline size of anatase phase nanocrystallites in the shells may be less than 10 nanometers.

[0083] The foregoing methods have been found to provide titania-based materials with enhanced thermal stability. Without wishing to be bound by any theory, it is believed that the enhanced thermal stability is due in part to the unique fabrication techniques in conjunction with use of silica as a functional support for titania nanoparticles. Thermally stable S1O2 supports may prevent uncontrolled crystallite growth and phase transformation of anatase nanoparticles by improving particle dispersion and by particle immobilization on the support via Si-O-Ti covalent attachment. For instance, such improved structural behavior of silica supported titania was observed on nanostructured Si02@Ti0 ₂ core-shell particles prepared by sol-gel deposition of titania shells on S1O2 spherical particles.

[0084] FIG. 2 illustrates several illustrative processes for sol-gel synthesis of

SiC /TiC aerogels.

[0085] Preparation of iOi aerogel

[0086] The illustrative process of S1O2 aerogel preparation 200 is a comparative example involving gelation of a mixture 202 comprising TEOS:Ethanol:HCl in NH4OH to form a Si02 wet gel 204. Supercritical CO2 drying of a S1O2 wet gel 204 forms a S1O2 aerogel 206.

[0087] Preparation of T1O2 aerogel

[0088] The illustrative process of T1O2 aerogel preparation 210 is a comparative example involving gelation of a titanium tetrachloride (TiCU) sol mixture 212 of TiCU + H2O in dimethylformamide (DMF) in propylene oxide to form a T1O2 wet gel 214.

Supercritical CO2 drying of a T1O2 wet gel 214 forms a T1O2 aerogel 216. In various approaches, the TiCU concentration may vary, and the watenTiCU ratio is preferably maintained at a ratio of about 6: 1.

[0089] Preparation of Si02-Ti02 composite aerogel following EAG process

[0090] The illustrative process of S1O2 dried aerogel plus TiCU Epoxide assisted gelification (EAG) 220 involved S1O2 dried aerogel powder (particles) 222 dispersed in TiCU sol mixture 212 (TiCU + H ₂ 0 in DMF). The mixture is combined, stirred, magnetic stirred, etc. for a selected time duration, preferably greater than 12 hours, e.g., about 18, 24, 48 hours, etc. Gelification may be induced by addition of DMF plus propylene oxide to form a Si02-Ti0 ₂ wet gel 224. The SiC -TiC wet gel 224 may be rinsed with DMF, ethanol, and acetone. Supercritical CO2 drying of the SiC -TiC wet gel 224 followed by calcination of the dried the SiC -TiC wet gel 224 at a temperature effective to cause titania crystallization, e.g., 600°C, 800°C, 1000°C, etc. for a duration effective to result in calcination, such as several hours (e.g. 1, 2, 4, etc. hours), may result in formation of a Si0 ₂ -Ti0 ₂ aerogel 226.

[0091] Preparation of Si02@Ti02 core-shell aerogel following TID process

[0092] The illustrative process of a wet S1O2 gel + TiCU thermohydrolysis route 230 includes formation of a S1O2 wet gel 204 that is modified by soaking the gel for a duration of time (for example, greater than 8 hours, e.g. 12, 24, 30, etc. hours) in a TiCU sol mixture 212 (TiCU + H2O in DMF) at an elevated temperature above room temperature, for example greater than 30°C, preferably greater than 60°C, e.g., 80°C. In preferred approaches, the temperature is sufficient to promote precipitation of T1O2 induced by thermohydrolysis of TiCU sol. A formed Si02@Ti02 core-shell wet gel 234 may be washed with DMF, ethanol, and acetone. Supercritical CO2 drying of the Si02@Ti02 core-shell wet gel 234 followed by calcination of the dried Si02@Ti02 core- shell wet gel 234 at a temperature effective to cause titania crystallization, e.g., 600°C, 800°C, 1000 °C for several hours (e.g. 1, 2, 4, etc. hours), thereby forming a Si0 ₂ @Ti0 ₂ core- shell aerogel 236.

[0093] Si02@Ti02 core-shell wet gels and aerogels [0094] FIGS. 3A-3D are images of prepared Si02@Ti0 ₂ core-shell wet gels and aerogels. Titania deposition on S1O2 supports (wet gels or aerogel particles) was achieved by TiCU non alkoxide sol-gel routes. T1O2 formation was induced by heating at 80°C (TIC route) . FIG. 3A shows Si0 ₂ @Ti0 ₂ core-shell wet gels with an increasing concentration TiCU. Tube 1 is the Si0 ₂ control without TiCU, and progressively increasing concentrations TiCU were added to tubes 2 through 6. As the concentration of TiCU increased, the translucence of the wet gel decreased and the wet gel become more opaque, indicating deposition of increasing titania content on the silica gel surface. FIGS. 3B-3D are images of monolithic forms of the Si0 ₂ @Ti0 ₂ core-shell aerogels were obtained after drying the wet gels in supercritical C0 ₂ .

[0095] FIG. 3E is an image of prepared Si0 ₂ -Ti0 ₂ composite wet gel formed by epoxide-assisted gelation (EAG route) using propylene oxide as proton scavenger. FIG. 3F is an image of a monolithic form of a Si0 ₂ -Ti0 ₂ composite aerogel obtained after drying the wet gel in supercritical C0 ₂ .

[0096] N2 Physisorption

[0097] FIGS. 4A and 4B are plots of N ₂ adsorption-desorption isotherms of

Si0 ₂ @Ti0 ₂ core-shell aerogel samples (FIG. 4A) and Si0 ₂ -Ti0 ₂ composite aerogel samples (FIG. 4B). Controlled drying avoided shrinkage and pore collapse, thereby preserving the fine pore network structure in the dried material, and thus resulting in low density mesoporous solids with high porosity and large specific surface area, as evidence by N ₂ physisorption measurements. FIG. 4A depicts N ₂ adsorption-desorption isotherms plots for Si0 ₂ @Ti0 ₂ core-shell aerogel samples (TID route) as prepared and heat treated at different temperatures. FIG. 4B depicts N ₂ adsorption-desorption isotherms plots for Si02-Ti0 ₂ composite aerogel (EAG route) samples as prepared and heat treated at different temperatures.

[0098] FIGS. 5A-5C depict plots that compare loss of specific surface area (in absolute values (as shown in part (a) )and percentages (as shown in part (b)) with increasing thermal treatment temperature (600°C, 800°C or 1000°C). FIG. 5A depicts plots of loss of specific surface area of Si0 ₂ @Ti0 ₂ core-shell aerogels (TID) with increasing temperatures. FIG. 5B depicts plots of loss of specific surface area of S1O2- T1O2 composite aerogels (EAG) with increasing temperatures. FIG. 5C depicts plots of loss of specific surface area of T1O2 aerogels with increasing temperatures.

[0099] N ₂ adsorption-desorption isotherms of both Si0 ₂ @Ti0 ₂ core-shell aerogels (TID) and Si0 ₂ -Ti0 ₂ composite aerogels (EAG) may be classified as type IV isotherms, which are characteristic of mesoporous materials, thus confirming the porous nature of the prepared aerogel photocatalysts. Nevertheless, slight differences are observed between the two different silica-titania aerogels in regard to their isotherm hysteresis shapes. Si0 ₂ @Ti0 ₂ core-shell aerogels isotherms show HI hysteresis loops, indicating the presence of uniform cylindrical mesopores, which is consistent for solids constituted by agglomerated spherical particles. On the other hand, Si0 ₂ -Ti0 ₂ composite aerogels isotherms depict hysteresis loops better classified as H3 type, characteristic of nonuniform slit-shaped pores. Either way, given their mesoporous structure, the as-prepared silica-titania aerogels show high specific surface area (around 600 m ² /g, as shown in Table 1), which is a desirable characteristic in high performance photocatalysts. Table 1. Textural properties of prepared aerogel materials

[00100] Analysis of the N ₂ adsorption isotherms, derived textural properties values, and thermal stability are shown in Table 1. Without wishing to be bound by any theory, the observation of no significant changes in the isotherms shape as function of annealing temperature may suggest that the overall pore structure is maintained even after high temperature thermal treatments. Upon annealing, specific surface area of the silica- titania aerogels decreases as one would expect considering that thermal treatments lead to processes such as titania crystallization and pore coalescence. Nevertheless, the loss of surface area observed for the thermally- stable silica-titania aerogels may be relatively much smaller compared to that shown by bare T1O2 aerogel (more than 99% after 800°C treatment) as shown in Figure 4C , further confirming the materials enhanced thermal stability.

[00101] X-ray diffraction analysis

[00102] FIGS. 6A-6C show powder X-ray diffractograms analysis of T1O2 aerogel and Si0 ₂ /Ti0 ₂ aerogels annealed at 600 °C, 800 °C, and 1000°C. FIG. 6A shows exclusive formation of anatase (A) nanocrystallites in the heat-treated Si02@ i02 core-shell aerogels, which demonstrate exceptionally high thermal stability as crystalline size did not increase past 10 nm even after the materials were submitted to 1000°C thermal treatment.

[00103] On the other hand in FIG. 6C, at high temperatures, as shown for 1000°C, an unsupported T1O2 aerogel sample undergoes crystallite growth (>100nm) and extensive rutile (R) formation, including total conversion of anatase phase into rutile phase.

[00104] SiC -TiC composite aerogels formed by the EAG route (FIG. 6B) shows predominately anatase (A) nanocrystallites with slightly higher anatase crystallite growth (from 7 nm to 14 nm) and small formation of rutile phase was observed for SiC -TiC composite aerogels (EAG) annealed at 1000°C compared to Si02@ i02 core-shell aerogels (TID). The anatase T1O2 nanocrystals supported on silica aerogel (FIGS. 6A and 6B) present remarkable thermal stability, remaining stable and not being converted into rutile even after calcination at 1000°C. Thus, as shown in FIGS. 6A and 6B, at higher temperatures, as shown for 1000°C, the Si /TiC aerogels have nanocrystallites of the anatase phase of T1O2 of a size less than 15 nm.

[00105] FIG 6D is an illustrative drawing of structural behavior of the aerogels following thermal treatment. Part (a) of FIG. 6D illustrates the change in structure of a Si02@Ti0 ₂ core-shell aerogels 502 formed by TID route following heat treatment. As shown prior to heat treatment, the structure of the Si02@ i02 core-shell aerogel 500 may include a S1O2 wet gel 504 with anatase phase T1O2 506 (A-T1O2) surrounding the S1O2 wet gel 504. Heat treatment at temperatures greater than 1000°C may cause a change of crystalline growth of the anatase phase T1O2 506 (A-T1O2) to a stable anatase phase in the Si02@ i02 core- shell aerogel 502. Moreover, Si02@ i02 core-shell aerogel 502 may comprise pure anatase phase T1O2 506 (A-T1O2).

[00106] Part (b) of FIG. 6D illustrates the change in structure of a Si -Ti

composite aerogel 512 formed by EAG route following heat treatment. As shown prior to heat treatment, the structure of the SiCh-TiCh aerogel 510 may include particles of S1O2 wet gel 504 surrounded with T1O2 506 (A-T1O2). Heat treatment at temperatures greater than 1000°C causes a change of crystalline growth of the anatase T1O2 506 (A-T1O2) to a mixture of anatase phase T1O2506 (A-T1O2) and rutile phase T1O2 508 (R- T1O2) in the SiCh-TiCh composite aerogels 512. Thus, the SiCh-TiCh composite aerogels 512 may demonstrate a partial anatase phase to rutile phase conversion of the T1O2.

[00107] Part (c) of FIG. 6D illustrates the change in structure of a T1O2 aerogels 518 following heat treatment. As shown prior to heat treatment, the structure of the T1O2 aerogel 516 may includes anatase T1O2 506 (A-T1O2). Heat treatment at temperatures greater than 800°C may cause a change of greater crystalline growth of the anatase T1O2 506 (A-T1O2) to a rutile phase T1O2 508 (R- T1O2) in the T1O2 aerogels 518. Moreover, the T1O2 aerogels 518 may demonstrate a full (e.g. complete) anatase phase to rutile phase conversion of the T1O2. Thus, the T1O2 aerogel 518 may comprise pure rutile phase T1O2 508 (R- Ti0 ₂ ).

[00108] Enhanced thermal stability of anatase nanocrystals in prepared silica-titania aerogels can be assigned to the role of silica as an effective thermal stable support. In the case of the Si02@ i02 core-shell aerogel (TID), the silica aerogel backbone act as an immobilizing support for T1O2 nanocrystals, avoiding its crystallite growth and rutile formation, while in the Si -Ti composite aerogel (EAG) silica particles are dispersed in titania network, thus probably minimizing sintering and phase transformation by acting as steric barriers to the diffusion of T1O2 nanoparticles.

[00109] Photocatalysis

[00110] FIGS. 7B-7D show photocatalytic degradation of Rhodamine B (RhB) dye in the presence of Si02@ i02 core-shell aerogels particles (TID route, FIG. 7B) and S1O2- T1O2 composite aerogels particles (EAG route, FIG. 7C) T1O2 aerogel (FIG. 7D) monitored by the decrease RhB concentration as a function of UV irradiation (FIG. 7A). Figure 7E compares the RhB photodegradation first-order kinetics constants of aerogels of Si02@Ti02, SiC -TiC and bare T1O2. Both the prepared Si02@Ti02 core-shell and SiC -TiC aerogels (FIG. 7B and 7C) showed remarkably higher photocatalytic activity as compared to unsupported T1O2 aerogels (FIG. 7D) as demonstrated by Rhodamine B photodegradation assays.

[00111] Improvement of the photocatalytic activity of Si /Ti aerogels was observed upon calcination at 800-1000 °C as shown in FIG. 7E. Without wishing to be bound by any theory, the increase in crystallinity may improve the photocatalytic activity of the aerogels. As shown in FIG. 7D, a drastic decrease in photoactivity was observed for calcined unsupported titania. Without wishing to be bound by any theory, it is believed the loss of photoactivity of the unsupported titania may be caused by complete conversion of anatase into rutile and significant loss of specific surface area.

[00112] Highest photocatalytic activity for RhB photodegradation was achieved for Si02-Ti0 ₂ composite aerogels with T1O2 content in the range of 55- 75% and calcined at 1000°C and for Si0 ₂ @Ti0 ₂ core shell aerogels with Ti0 ₂ content in the range of 55-65% and calcined at 1000°C. Further evaluation of the materials photocatalytic activity was carried out by Crystal Violet (CV) dye photodegration. FIG. 8 depicts a comparison of photocatalytic activity in terms of CV photodegradation first order kinetic constants for the most active Si0 ₂ @Ti0 ₂ core-shell aerogels and Si0 ₂ -Ti0 ₂ composite aerogels calcined at 1000°C with those of high performance commercial photocatalyst Degussa P25 before and after 1000°C thermal treatment. Both the calcined silica-titania aerogel show higher photocatalytic activity compared to P25 and vastly outperforms the calcined P25 samples. Without wishing to be bound by any theory, it is believed that superior photocatalytic performance of Si0 ₂ @Ti0 ₂ core-shell aerogels and Si0 ₂ -Ti0 ₂ composite aerogels may be assigned to the high photoactivity of silica supported anatase

nanocrystals as well as improved Rhodamine B adsorption capacity by Si0 ₂ aerogel support.

[00113] In Use

[00114] Various embodiments of the aerogels described herein may be used in catalysis, photocatalysis, photodegradation, purification, and/or any other conceivable application. For example, various embodiments are useful as a photocatalyst for energy and environmental applications such as organic pollutants photodegradation for air and water purification, photo-assisted removal of toxic heavy metals, and production of solar fuels by water splitting and CO2 reduction. Importantly, the materials show outstanding thermal stability and excellent photocatalytic activity even after 1000°C thermal treatment, making them interesting choices as photocatalysts for applications that demand high temperature processing such as self-cleaning coatings and photocatalytic ceramic tiles. Additionally, the developed non-alkoxide routes employ TiCU, a cheaper precursor as compared to commonly employed titanium alkoxides, and which allow coating of S1O2 monolithic gels in one-step deposition (preparation of Si02@Ti0 ₂ core-shell gels), making it more interesting than the previously report deposition methods that require several deposition cycles to achieve the desirable titania content.

[00115] The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, embodiments, and/or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in any combination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the art upon reading the instant descriptions should also be considered within the scope of this disclosure.

[00116] While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.