GOVERNMENT SUPPORT

This invention was made with government support under Grant No. DE- SG0001088 awarded by the U.S. Department of Energy. The U.S. Government has certain rights in the invention.

BACKGROUND

Aerogels have various applications due to their high surface area and low densities. Creating aerogels from various materials, however, has remained a challenge. For example, to date, only limited types of materials have been made into "aerogel" structures. These materials include metal oxide aerogels ( e.g ., S1O2 and AI2O3), carbon material aerogels [such as carbon, carbon nano tubes (CNTs), and graphene]; and, more recently, semiconducting chalcogenide aerogels (e.g., GdS, GdSe, and PbTe).

One reason that the types of usable materials to form aerogels are limited is the challenge of forming the starting "gel." Most aerogels are obtained through a sol- gel process with a suitable gelling agent precursor. For example, in the case of S1O2 aerogel, a liquid alcohol (e.g, ethanol) is mixed with a silicon alkoxide precursor, \e.g., tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS)]. A hydrolysis reaction forms particles of silicon dioxide, which may form a sol solution. The oxide suspension then undergoes condensation reactions, which result in the creation of metal oxide bridges (M-O-M bridges or M-OH-M bridges) linking the dispersed colloidal particles. When this interlinking has stopped the flow of liquid within the material, a gel is made. Carbon aerogels are made by subjecting gel precursor to supercritical drying and subsequent pyrolysis of an RF aerogel at high temperature. Because this cross-linking reaction is specific only to a selected group of materials, the number of materials that may be used to form aerogels is limited.

U.S. Patent No. 9,208,919 B2 is directed to methods for fabricating aerogel from a variety of materials. In a method recited therein, fabrication of the aerogel includes the following steps: (A) increasing a concentration of a suspension comprising a gel precursor under a condition that promotes formation of a gel, wherein the gel precursor comprises particulates having an asymmetric geometry; and (B) removing a liquid from the gel to form an aerogel, wherein the aerogel and the gel have substantially the same geometry. In another method recited therein, fabrication of the aerogel includes the following steps: (A) subjecting a suspension comprising a gel precursor comprising particulates to sonication and/ or filtering; (B) forming the suspension into a gel using hydro-thermal synthesis; and (G) removing a liquid from the gel to form an aerogel, wherein at least some of the particulates have an aspect ratio of at least 50.

SUMMARY

A method for producing an aerogel using a salt and the resulting aerogel are described herein, where various embodiments of the product and methods may include some or all of the elements, features and steps described below.

Traditionally, hydrothermal growth of an aerogel involved simply putting all of the reactants into the cavity and starting the reaction. However, for solid-based growth, such as T1O2 aerogel growth from photo catalytic standard 25 (P25) T1O2 nanoparticles, the source materials are not soluble in water, the particles tend to sink to the bottom and it’s hard to control the density of the as-grown aerogel.

In this disclosure, we introduced non-reactive salts into the cavity and successfully decreased the density of the resulting aerogel by three fold. This method adds another dimension for controlling the solid-based hydrogel/ aerogel growth. Furthermore, the resulting aerogel can be produced with lower density and better pressure performance (with a reduced pressure difference across a filter formed of the aerogel) in comparison with an aerogel from a similar process but without an added salt.

In accordance with embodiments, described herein, an aerogel can be produced by forming a gel precursor, comprising (a) a liquid medium, (b) a salt dissolved in the liquid medium, and (c) particulates suspended in the liquid medium, wherein the salt increases the density of the liquid medium. The gel precursor is heated to form a hydrogel from the particulates in the liquid medium via hydrothermal synthesis. The liquid medium can then be removed from the gel to form the aerogel. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of hydrogels 12 made from a solution with no KG1 salt (left) and 5M of KG1 salt (right).

FIG. 2 is a schematic illustration of hydrogel formation showing, from left-to- right: (a) insoluble particles 16 dispersed in solution in a liquid medium 14 in which salt 15 is dissolved, (b) nanowires 18 growing out of the particles 16, (c) nano wires 18 continuing to grow and becoming connected, and (d) formation of a hydrogel 12 in the form of a three-dimensional (3D) network of nanowires 18.

In the accompanying drawings, like reference characters refer to the same or similar parts throughout the different views; and apostrophes are used to

differentiate multiple instances of the same item or different embodiments of items sharing the same reference numeral. The drawings are not necessarily to scale;

instead, an emphasis is placed upon illustrating particular principles in the exemplifications discussed below. For any drawings that include text (words, reference characters, and/ or numbers), alternative versions of the drawings without the text are to be understood as being part of this disclosure; and formal

replacement drawings without such text may be substituted therefor.

DETAIFED DESCRIPTION

The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

Unless otherwise herein defined, used or characterized, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially (though not perfectly) pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities ( e.g. , at less than 1 or 2%) can be understood as being within the scope of the description. Likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to manufacturing

tolerances. Percentages or concentrations expressed herein can be in terms of weight or volume. Processes, procedures and phenomena described below can occur at ambient pressure {e.g., about 50-120 kPa— for example, about 90-110 kPa) and temperature {e.g., -20 to 50°G— for example, about 10-35°G) unless otherwise specified.

Although the terms, first, second, third, etc., may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are simply used to distinguish one element from another. Thus, a first element, discussed below, could be termed a second element without departing from the teachings of the exemplary embodiments.

Spatially relative terms, such as“above,”“below,”“left,”“right,”“in front,” “behind,” and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms, as well as the illustrated configurations, are intended to encompass different orientations of the apparatus in use or operation in addition to the orientations described herein and depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as“below” or“beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term,“above,” may encompass both an orientation of above and below. The apparatus may be otherwise oriented {e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. The term,“about,” means within ± 10% of the value recited. In addition, where a range of values is provided, each subrange and each individual value between the upper and lower ends of the range is contemplated and therefore disclosed. Further still, in this disclosure, when an element is referred to as being“on,” “connected to,”“coupled to,”“in contact with,” etc., another element, it may be directly on, connected to, coupled to, or in contact with the other element or intervening elements may be present unless otherwise specified.

The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, singular forms, such as“a” and“an,” are intended to include the plural forms as well, unless the context indicates otherwise. Additionally, the terms,“includes,” “including,”“comprises” and“comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps.

Additionally, the various components identified herein can be provided in an assembled and finished form; or some or all of the components can be packaged together and marketed as a kit with instructions ( e.g ;, in written, video or audio form) for assembly and/ or modification by a customer to produce a finished product.

In one aspect, a method of making a composition comprising an aerogel is provided. The method may include adding a salt and increasing a concentration of a suspension comprising a gel precursor under a condition that promotes formation of a hydrogel. Subsequently, the liquid in the hydrogel may be removed such that an aerogel is formed. In some embodiments provided herein, the hydrogel and the aerogel may have substantially the same geometry.

Adding a non-reactive salt 15 that dissolves into the liquid medium 14 increases the density of the liquid medium 14 and, therefore, reduces the propensity of denser 3-D gel networks 12 to settle toward the bottom of the liquid medium 14, as shown in FIG. 1. The non-reactive salt 15 (e.g., a potassium or sodium salt) can be added to reach a concentration (at ambient/room temperature) up to its saturation point (to prevent salt crystallization). The salt concentration can be, e.g, in a range from 50% of its saturation point up to the saturation point.

In accord with the methods described herein, and as schematically shown in the left-most image of FIG. 2, the hydrogel 12 can be formed by dispersing insoluble particles 16 in the liquid medium 14 in which the salt 15 is dissolved. As the temperature and pressure of the liquid medium 14 is raised, nanowires 18 grow out of the particles 16, as shown in the next image of FIG. 2. As shown in the following image, the nanowires continue to grow and inter-connect. Finally, as shown in the right-most image of FIG. 2, a hydrogel 12 is produced in the form of a 3D network of the inter-connected nanowires 18.

The particles 16 can have a composition selected from, e.g., T1O2, V2O3, or graphene and can have a particle size of, e.g., 10 nm to 100 pm. The liquid medium 14 can include water and either (a) an organic precursor (that includes the structural composition of the hydrogel) and a reactant that releases the structural composition to form the hydrogel or (b) a reactant {e.g., a hydroxide, such as KOH or NaOH) that reacts with T1O2 to produce the structural composition {e.g., K2T18O17) of the hydrogel. Where the structural composition is formed from T1O2, the organic precursor can be, e.g., titanium isopropoxide, Ti{OCH(CH3)2}4, and the reactant can be sulfuric acid.

Gel Precursor:

The gel precursor described herein may be any of a wide variety of materials, depending on the type of aerogel desired. The methods described herein are versatile and may be employed to make a variety of types of aerogel material. For example, the precursor may contain a metal, a compound, a semiconductor, a carbon-containing material, or combinations thereof. One feature of at least one embodiment described herein is that the methods described herein allow gel (and, finally, aerogel) to be formed with a relative low concentration of the precursor material.

The metal may be any metal, including noble metal and transition metal. For example, a noble metal may be gold, silver, platinum, copper, and the like. A transition metal may be any element in Groups 3-12 of the Periodic Table. The term, "element," as used herein, refers to the elements found on the Periodic Table. For example, a transition metal may be Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Gr, Mo, W,

Sg, M, Tc, Re, Bg, Fe, Ru, Os, Hs, Go, Rh, Ir, Mt, Ni, Pd, Pt, Ds, Gu, Ag, Au, Rg, Zn, Gd, Hg, Gn. In some embodiments, the metal may be silver. The compound may refer to any of a variety of compounds depending on the applications. For example, the compound may be an oxide, a nitride, a

chalcogenide, and the like. An oxide may be a metal oxide ( e.g ;, alumina, titania, iron oxide, zinc oxide, manganese oxide, alkali-metal oxide, alkali-earth metal oxide, or any of the metals described above). The oxide may alternatively be a non-metal oxide, including silica. The compound may alternatively be a metal nitride, metal sulfide, including any of the aforementioned metals as the metal element. For example, the compound may be M0S2 , GdS, GdSe, Pb Te, or combinations thereof. Alternatively, the nitride and sulfide may be a non-metal nitride and sulfide. For example, the compound may be a boron nitride (e.g., hexagonal boron nitride, or "h- BN").

The semiconductor may be selected from any known semiconductors. The semiconductor may be an elemental semiconductor (only one element) or a compound semiconductor (more than one element). For example, the

semiconductor may be silicon. Alternatively, the semiconductor material may be GaAs, GaN, MnCh, T1O2, ZnO, BbTes, or combinations thereof.

The carbon-containing material may be any known structure that contains carbon atoms and can be provided in a powder form. For example, the material may be graphite, carbon nanotube, carbon nanowire, or graphene. The carbon

nanotubes may be single -walled carbon nanotubes, multi-walled carbon nanotubes, or both.

The gel precursor may contain a plurality of particulates. The term

"particulates" may have any geometry and need not be spherical. The particulates described herein may have an asymmetric geometry (e.g, anisotropy) such that one dimension thereof is greater than the other; the dimensions described herein may refer to the diameter, length, width, and height of the particulate. One feature of at least some embodiments described herein is the formation of aerogels using one dimensional (1-D) and/ or two-dimensional (2-D) materials using a general principle of gel formation based on shape asymmetry. For example, the particulates may be wire-like, tube-like (i.e., wire-like but hollow), sheet-like, flake -like, or any other shape. The wire-like and tube-like particulates are herein regarded as being one- dimensional, while the sheet-like and flake-like particulats are herein regarded as being two-dimensionsl (with no more than a few atomic layers thickness). Because of the nanometer-length scale, in some embodiments, the particulates may be referred to as nanotubes, nanowires, or nanosheets, depending on the geometry; the particulates may comprise any of the afore-described materials.

The asymmetry may be described by, for example, an aspect ratio, which, in one embodiment herein, may refer to a ratio of the length to the diameter of a particulate (for a tubular-/ wire-like configuration) or to a ratio of the width or length to the thickness of a particulate (for a sheet-like configuration). Accordingly, the particulates may have an aspect ratio of greater than about 1— e.g., greater than about 10, about 50, about 100, about 150, about 200, about 250, about 300, about 350, about 400, about 450, about 500, about 550, about 600, about 650, about 700, about 750, about 800, about 850, about 900, about 950, about 1000, about 2000, about 5000, about 1,0000, or more. The aspect ratio may be higher (towards infinity) or lower (towards 1.1) than the aforedescribed values. For example, the aspect ratio may be between about 1 and about 1000— e.g., between about 10 and about 500, between about 100 and about 400, or between about 200 and about 300.

The particulates may have any size, ranging from nanometers to microns. The size may refer to an average size in the case of a plurality of particulates. The size may refer to any dimension, including length, width, height, thickness, diameter, etc., depending on the geometry. In some embodiments, the diameter of the particulate described herein may be less than about 500 nm— e.g., less than about 400 nm, about 300 nm, about 200 nm, about 100 nm, about 50 nm, about 20 nm, about 10 nm, about 5 nm, about 1 nm, or less. For example, the diameter may be between about 10 nm and about 500 nm— e.g., between about 20 nm and about 400 nm, between about 50 nm and about 300 nm, or between about 100 nm and about 200 nm. Other dimensions, including the length, of the particulates may be calculated by the aspect ratio described above. For example, the length may be at least about 0.5 microns— e.g., at least about 1 micron, about 2 microns, about 4 microns, about 8 microns, about 16 microns, about 32 microns, or more. In some embodiments, the particulates contain silver and have an average diameter of about 113 nm and an average length of about 13.7 mhi. In some other embodiments, the particulates contain silicon and have an average diameter of about 41 nm and an average length of about 5.2 pm. In some other embodiments, the particulates contain manganese oxide and have an average diameter of about 19 nm and an average length of about 8.4 pm.

A salt, such as a potassium salt, is added to the gel precursor. The salt increases the density of the liquid in the gel precursor. Examples of high-solubility potassium salts that can be dissolved in water [at or near room temperature and 1- atmosphere pressure (~ 101 kPa)] up to the limits of their solubility and used in the gel precursor are listed in Table 1, below, where units of solubility are given in grams per 100 milliliters of water (g/100 ml).

Table 1 :

The salt can be added up to the saturation point of the salt in the solution at room temperature. By avoiding exceeding the saturation point at room temperate, precipitation of the salt can be avoided. The effect of salt (KGL) density on the density of the resulting aerogel for a sample in the form of a disc with a diameter of 1 inch (2.54 cm) and a thickness of 4 mm in a 10M KOH solution is shown in Table 2, as follows: Table2:

Accordingly, titania (T1O2) aerogels with a density less than about 41.9, 30.1, 21.2, or 17.3 mg/cm ³ can be formed by adding, respectively, at least 2, 3, 4, or 5 M of salt in the formation of the hydrogel.

In one embodiment, we take hydrogel/aerogel growth from T1O2 (hydrogels and aerogels formed from T1O2 precursor may be herein referred to as T1O2 hydrogel/ aerogel) as an example. For a typical process, P25 T1O2 nanoparticles are mixed into 10M KOH solution and subject to a 180~250°C hydrothermal reaction to form T1O2 hydrogel 12 ( e.g ;, via the following reaction: K2T18O17 +H2O) in a liquid medium 14, as shown in FIG. 1 (left). The hydrogel is then dried to produce aerogel. However, the density of hydrogel/ aerogel is hard to control for a given density of KOH solution, which determines the nanostructure of the hydrogel/aerogel. Hence, we choose KG1 as an example of high-solubility potassium salt (Table 1) to add into the solution, which endows a larger floating force for nanoparticles and thus produces a T1O2 hydrogel in FIG. 1 (right). Finally, the hydrogels are freeze-dried to produce aerogels. For a 1-inch, 4-mm thick sample, a no-KGl sample weights 100 mg and 5M KG1 sample weights 35 mg. The densities are 49.34 mg/cm ³ for the sample without salt and 17.27 mg/cm ³ for the sample with salt.

In other embodiments, other compositions, such as V2O3 or graphene, can be used in place of T1O2.

In other embodiments, where particulates are added to a NaOH solution rather than a KOH solution, sodium salt can be used in place of potassium salt. In still other embodiments, the salt is selected from at least one of a calcium salt, a lithium salt, a magnesium salt, a titanium salt, a manganese salt, a molybdenum salt, a tungsten salt, and an iron salt.

The particulates of the aerogels may be made using chemical vapor deposition (GVD), physical vapor deposition (PVD), and or hydrothermal or electrochemical deposition in anodic aluminum oxide (AAO) template. In some embodiments, hydrothermal synthesis may refer to a method of crystallizing a substance from hot water under high pressure. The temperature of the water may be at about 50° G or more— e.g., about 60° G, about 70° G, about 80° G, about 90° G, or more. The pressure may be at least about 2 atm (~203 kPa)— e.g., about 3 atm (~304 kPa), about 4 atm (~405 kPa), about 5 atm (~507 kPa), or more. Subsequently, gels may be formed from the method described below. In one embodiment, the nanowires synthesis may involve at least one of hydrothermal synthesis of MnCh and/ or T1O2 , GVD synthesis of ZnO and/ or GaN, and electrochemical

etching/deposition of Si, BbTes, Ag, Pt, and/or Au nanowires.

Gel Formation:

As described above, the hydrogels formed by the methods described herein may be employed to form aerogels. The hydrogel formation may be tailored by controlling a reaction time of the synthesis. The synthesis may involve, for example, hydrothermal synthesis.

The reaction time may be tailored to be of any length of time, depending at least on the materials involved. For example, the reaction time may be at least about 5 minutes— e.g., at least about 10 minutes, about 20 minutes, about 30 minutes, about 60 minutes, about 2 hours, about 4 hours, about 6 hours, about 8 hours, about 10 hours, about 20 hours, about 40 hours, about 50 hours, about 60 hours, about 100 hours, about 120 hours, or longer.

In some embodiments, the methods described herein may further include adding chemical coatings {e.g., polymer electrolyte) directly to the gel (skeleton) before the liquids are extracted from the hdrogel to form an aerogel. In the case of T1O2 nanowire, dye-sensitized solar cell ("DSSC") devices may be constructed. The method may start with gel formation using T1O2 nanowire, and the gel may be made into a thin film on a glass slide. By soaking the T1O2 gel into a N-719-dye solution, a uniform layer of dye will be coated on the nanowire surface. Since the nanowire network is already formed, the contact region between the nanowires will not be coated so that the direct contact between the nanowires will be ensured.

Afterwards, the gel is soaked into a solution of polyethylene oxide (PEO) with KI/I2; the PEO with KI/I2 will be the electrolyte layer for the DSSG (a different solution). Then, electrodeposition of a thin layer (5-10 nm) of Pt is carried out, so the nanowires are coated with Pt serving as cathode. Finally, super-critical-point drying (GPD) is carried out to obtain the coated aerogel. For all of these coating steps, only part ( e.g ., 80%) of the gel is coated, with one side of the gel region having bare T1O2 nanowires; this side with the bare T1O2 nanowires is contacted and serves as the anode. Those nanowires having exposed sections in this region are directly contacted to the anode; the rest of the nanowires are contacted through the nanowire network to the anode.

Aerogel Formation:

The hydrogel formed according to the methods described above may be further dried to remove the liquid (solvent) from the hydrogel to form an aerogel. Drying may be carried out by any suitable drying techniques, depending on the materials involved. The techniques may include (i) freeze drying, (ii) supercritical- point drying ("GPD"), or both.

In one alternative embodiment, the aerogel may be formed by a method comprising: (A) subjecting a suspension comprising a gel precursor comprising particulates to at least one of ultrasonication and filtering; (B) forming the suspension into a gel using hydrothermal synthesis; and (G) removing a liquid from the gel to form an aerogel. The precursor and gel may be any of those

aforedescribed.

In a GPD process in one embodiment, the liquid may be dried off slowly without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation techniques. As a result, the 3-D structure of the particulates in the gel may be preserved in the aerogel upon the transition from gel into an aerogel. For example, the aerogel may contain a 3-D network of crystalline nanowires, nanosheets, nanotubes, or combinations thereof. In some embodiments, the level of preservation may account for minute discrepancies, so long as at least the majority ( e.g ., substantially all, or even all) of the network structure is preserved. In some embodiments, because of this preservation, the geometry of the gel may also be preserved upon the transition into the aerogel. The geometry in some embodiments herein may refer to shape, size (e.g., volume), and the like.

Because the nanowires may be synthesized by a hydrothermal method, the gel formation may take place during the hydrothermal synthesis when the nanowires are crystallizing out from the hot water under high pressure. In one embodiment, the method may include forming the gel from the gel precursor by hydrothermal synthesis. The diameter and the length of the nanowires may depend on the pH and concentration of the solution, the temperature, and the reaction time. In one embodiment, by fixing the reaction conditions but changing the reaction time, nanowire gels of different densities (and porosities) may be obtained.

The aerogels produced according to the methods described in some embodiments herein may have desirable properties, including high surface areas and high thermal resistivity. The aerogel may be hydrophobic or hydrophilic. In one embodiment, a portion of the aerogel is hydrophilic and another portion thereof is hydrophobic. The aerogel may be elastic; in some embodiments, the aerogel exhibits superelasticity. The aerogels described herein may have a much higher electrical conductivity than an aerogel produced by a conventional technique. For example, the presently described aerogels may have an electrical conductivity that is larger than a conventional aerogel by a factor of at least about 2, about 3, about 4, about 6, about 8, about 10, or more. In some embodiments, the aerogel may have an electrical conductivity that is at least about 200 S/ m— e.g, at least about 300 S/ m, about 400 S/m, about 600 S/m, about 800 S/m; about 1,000 S/m; about 0.5 x 10 ⁴ S/m; about 1 x 10 ⁴ S/m; about 0.5 x 10 ⁵ S/ m; about 1 x 10 ⁵ S/ m; about 0.5 x 10 ⁶ S/ m; about 1 x 10 ⁶ S/m; about 0.5 x 10 ⁷ S/m; about 1 x 10 ⁷ S/m, or more. In one embodiment, the aerogel has an electrical conductivity of at least 3 x 10 ⁶ S/ m. The aerogels produced according to the methods described in some embodiments herein may have a mesoporous microstructure, having high porosity and/ or high surface area (i.e., low density). The mesoporous microstructure may be interconnected. The pores may have any geometries. In some embodiments, the pores may be cylindrical, slit-shaped, or any other shape, or a combination of any of these. For an aerogel sample produced from a 0M KG1 solution, the pore size is 11.85nm, while the pore sized for an aerogel sample produced from a 5M KG1 solution is 15.48nm.

As described above, the properties of the aerogels may depend on the materials involved. For example, when the particulates contain silver, the aerogel may have (i) an electrical conductivity of at least about 3 x 10 ⁶ S/m, (ii) a density of less than or equal to about 90 mg/ cm ³ , or both. Alternatively, when the particulates contain single -wall carbon nanotubes, the aerogels may have (i) an electrical conductivity of at least about 300 S/m, (ii) a density of less than or equal to about 2.7 mg/ cm ³ , or both. Alternatively, when the particulates contain graphene, the aerogel may have (i) an electrical conductivity of at least about 400 S/ m, (ii) a density ofless than or equal to about 15 mg/cm ³ , or both.

Because of the aforedescribed desirable properties, aerogels described herein may be used in applications including filtration, catalysis, sensing, energy storage, solar cells, fuel cells, thermal insulation, ultra-light structural media, and many other applications. For example, the aerogel may be a part of an electronic component (of an electronic device). In some embodiments, the electronic component may be a capacitor, including a super-capacitor. In a particular aplicaiton, the afore-described aerogels produced via the foregoing methods can be used as a filter for catalysis and filtration of a fluid stream with entrained solids, wherein the aerogel can trap the solids as the fluid passes through the filter and can catalyzed reactions of

components in the fluid. Such a filter can be used for a variety of applications, such as for removing viruses, bacteria, allergens, dust, combustion products, etc.

Specifically, such a filter can be used, e.g., in a factory (e.g., positioned in the exhaust stream of a smokestack); in a motor vehicle (e.g, positioned in the engine exhaust stream leading to the tailpipe); or in the home, a building, a hospital, or a clean room ( e.g ., in a semiconductor fabrication lab or in a lab where biological processes are carried out) in an air-intake flow stream to provide clean air to any of these environments.

Additional examples consistent with the present teachings are set out in the following numbered clauses:

1. A method of producing an aerogel, comprising:

forming a gel precursor, comprising (a) a liquid medium, (b) a salt dissolved in the liquid medium, and (c) particulates suspended in the liquid medium, wherein the salt increases the density of the liquid medium;

heating the gel precursor to grow a hydrogel from the particulates in the liquid medium via hydrothermal synthesis; and

removing the liquid medium from the hydrogel to form the aerogel.

2. The method of clause 1, wherein the salt is selected from at least one of the following types of salts: potassium salts and sodium salts.

3. The method of clause 1 or 2, wherein the particulates comprise a composition selected from T1O2, V2O3, and graphene.

4. The method of clause 3, wherein the particulates comprise T1O2.

5. The method of any of clauses 1-4, wherein the liquid medium comprises

water.

6. The method of any of clauses 1-5, wherein the salt can dissolve in the liquid medium up to a saturation limit, and wherein the salt is dissolved in the liquid medium at a concentration in a range from half of the saturation limit up to the saturation limit.

7. The method of any of clauses 1-6, further comprising passing a fluid stream through the aerogel to filter particles entrained in the fluid stream and to catalyze reactions of components in the fluid stream.

In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step.

Likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties or other values are specified herein for embodiments of the invention, those parameters or values can be adjusted up or down by l/100 ^th , l/50 ^th , l/20 ^th , l /10 ^th ,

1 /5 ^th , 1 / 3 ^rd , 1/2, 2/3 ^rd , 3/ 4 ^th , 4/5 ^th , 9/10 ^th , 19/20 ^th , 49/50 ^th , 99/100 ^th , etc. (or up by a factor of 1 , 2, 3, 4, 5, 6, 8, 10, 20, 50, 100, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions, and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of references, including reference texts, journal articles, patents, patent applications, etc., cited throughout the text are hereby incorporated by reference in their entirety for all purposes; and all appropriate combinations of embodiments, features, characterizations, and methods from these references and the present disclosure may be included in embodiments of this invention. Still further, the components and steps identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims (or where methods are elsewhere recited), where stages are recited in a particular order— with or without sequenced prefacing characters added for ease of

reference— the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.