Polyoxometalate (POM)-complexed metal-oxide nanocrystal GELS and use thereof

Abstract

Gels and aerogels are provided that comprise one or more polyoxometalate (POM)– complexed nanocrystal cores formed from metal oxides, metal hydroxides, and / or metal oxyhydroxides. Coordination of the nanocrystal surfaces by POM cluster anions imparts enhanced structural stability, surface accessibility, and electronic tunability relative to non- complexed materials. As a result, the POM-complexed nanocrystals exhibit improved performance in chemical, electrochemical, photochemical, and photoelectrochemical processes. Conversion of the POM–nanocrystal complexes into gel and aerogel architectures further enhances reactivity and functional versatility by providing interconnected, high-surface- area networks that retain exposed, catalytically active inorganic surfaces.

Description

[0001] POLYOXOMETALATE (POM)-COMPLEXED METAL-OXIDE NANOCRYSTAL GELS AND USE THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to polyoxometalate (POM)-complexed inorganic nanocrystal materials, particularly, but not exclusively, to assembly of POM-ligated metal-oxide nanocrystals to gels and aerogels, and use thereof in chemical, catalytic, electrochemical, photochemical, and photoelectrochemical applications.

[0004] BACKGROUND

[0005] Supramolecular and nanoparticle-based gels constitute an important class of functional materials with applications spanning catalysis, optics, electronics, and electrochemistry. Metal-oxide nanocrystal (NC) gels are of particular interest because they combine high surface area with tunable electronic and chemical properties. However, practical utilization of such materials requires reliable strategies for assembling nanocrystals into extended, mechanically stable networks while preserving their intrinsic structure and functionality. Conventional sol-gel methods are limited in this respect, as they rely on irreversible chemical transformations of metal precursors that frequently produce amorphous materials or poorly controlled crystalline phases, with limited control over nanocrystal size, crystallinity, and composition.

[0006] An alternative approach would be the use of pre-formed nanocrystals as building blocks for gel formation. Because the nanocrystals are synthesized prior to assembly, their size, composition, and crystal structure can be precisely defined. At sufficiently high concentrations, these nanocrystals can self-assemble through non-covalent interactions into three-dimensional networks that immobilize solvent to form gels, while retaining the optical, catalytic, electronic, and chemical properties of the individual nanocrystals.

[0007] Polyoxometalates (POMs) are a diverse class of polynuclear metal-oxo cluster anions, commonly derived from early transition metals in high oxidation states. POMs exhibit well-defined molecular structures and rich redox, catalytic, and coordination chemistry. Polyoxometalate-nanocrystal (POM-NC) complexes are known hybrid inorganic materials in which polyoxometalate (POM) cluster anions act as robust, multidentate inorganic ligands bound to the surfaces ofcrystalline metal-oxide and metal hydroxide nanocrystals. In such complexes, POMs stabilize and functionalize metal-oxide surfaces as well as coordinate transition-metal and rare-earth cations.

[0008] Prior publications by the present inventors (e.g., U. S. Patent Application Publication No.

[0009] 2025-0205689 and International Application Publication No. WO 2025 / 186767) have demonstrated that POM ligation provides exceptional colloidal stability while preserving access to catalytically active metal-oxide surfaces, thereby overcoming limitations associated with both electrostatically stabilized colloids and organic-ligand-capped nanocrystals. These complexes exhibit tunable surface charge, controlled protonation, enhanced redox activity, and improved resistance to aggregation and precipitation across a wide pH range.

[0010] SUMMARY

[0011] A key discovery underlying the present disclosure is that concentrated aqueous solutions of POM-complexed metal-oxide nanocrystals spontaneously and reversibly formed a new class of self-healing gels. These gels are distinguished by their ability to undergo reversible cation exchange and to accommodate finely tuned pre-synthetic or post-synthetic modifications that directly control gel structure, chemical composition, and catalytic reactivity. Unlike conventional nanoparticle gels or polymer-crosslinked networks, these materials are entirely inorganic and free of organic ligands or binders.

[0012] Experimental evidence indicates that gelation was a general and reproducible property of POM-complexed nanocrystals across a wide range of metal-oxide and metal hydroxide cores and countercation identities. Gels disclosed herein formed spontaneously across a wide range of systems, including nanocrystals comprising oxides and hydroxides of main-group metals, transition metals, and lanthanide metals. Gel formation was observed when the nanocrystals were complexed with different types of POM ligands and occurred with a high tolerance to nanocrystal size dispersity. Accordingly, the gel-forming behavior was not limited by nanocrystal composition, ligand identity, or particle size uniformity.

[0013] Gel formation was equilibrium-controlled and is tentatively attributed to a self-limiting assembly mechanism in which negatively charged POM-NC complexes organize into thin, percolated walls. These walls enclose interconnected, water-filled nanocavities, producing acontinuous porous network rather than precipitated solids. Electrostatic repulsion between like-charged complexes, balanced by counterion redistribution and osmotic stabilization by confined water, limits wall thickness and arrests further growth.

[0014] From a mechanistic standpoint, the self-limiting nature of gel formation appears to arise from the presence of multiple counterions associated with each polyionic POM-NC complex. This multivalent ionic environment promotes localized charge compensation while preventing complete neutralization, thereby enforcing controlled assembly into extended but finite suprastructures. More broadly, this behavior represents a previously unrecognized design principle for engineering functional nanoparticle assemblies, in which polyionic building blocks self-organize into stable, porous, and reconfigurable materials.

[0015] Collectively, these findings establish POM-complexed metal-oxide nanocrystal gels as a new class of inorganic porous materials that fully exploit the high intrinsic reactivity of nanometric oxide surfaces while enabling unprecedented control over composition, structure, and function through reversible, ion-mediated assembly.

[0016] Further discovery underlining the present disclosure is that gel-wall structures became kinetically trapped upon replacement of water with ethanol, enabling conversion of the hydrogels into aerogels by critical-point drying while preserving the internal architecture. The precursor gels may be functionalized with a degree of compositional control and spatial homogeneity not previously achievable, including uniform incorporation of transition metal (TM) cations and the use of binary-NC gels as precursors to homogeneously mixed heterojunction aerogels. These materials provide new modes of interaction between metal-oxide nanocrystals and TM cations and constitute a new class of catalysts for metal-oxide-activated chemical, photochemical, electrochemical, and photoelectrocatalytic transformations of small molecules.

[0017] Aspects of the present disclosure relate to a gel and aerogel, each comprising one or more polyoxometalate-complexed nanocrystals of the Formula (I): Ql[-POM]mHf[NC], as defined herein.

[0018] The gels and / or aerogel disclosed herein are applied in a chemical, electrochemical, photochemical, or photoelectrochemical process, whereby they are contacted with a substance and induce, catalyze, promote and / or modulate a chemical or physicochemical transformation of the substance.The process is at least one of oxidation, reduction, redox cycling, oxygen activation, reactive oxygen species generation, reactive oxygen species quenching, hydrogen evolution, water oxidation and / or carbon dioxide conversion, optionally carried out at near-neutral pH, and the substance being reacted upon may be water, oxygen, hydrogen peroxide, carbon dioxide, nitrogen, an organic compound, or any combination thereof.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Some embodiments of the present disclosure are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments disclosed herein. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments described herein may be practiced.

[0021] In the drawings:

[0022] Figs. 1A-1H are Cryo-SEM images of various is cryogenic scanning electron microscopy (cryo-SEM) image of gels of various POM-complexed metal-oxide nanocrystals (NCs). 1A: Keggin-ion complexed 10-nm anatase-TiO2NCs (Li+salt); 1B: Keggin-ion complexed 2.7-nm ε-MnO2NCs (Li+salt); 1C: Keggin-ion complexed 1.5-nm FeCrO3NCs (Li+salt); 1D: Keggin-ion complexed 2.5-nm SnO2NCs (Li+salt); 1E Keggin-ion complexed 4.3-nm CeO2NCs (H+form); 1F: hexaniobate-ion complexed 1.6-nm photo-luminesce4nt Eu(OH)3NCs (2H+, 6Na+); 1G: hexaniobate-ion complexed 3.5-nm SnO2(5H+, 3K+); 1H: a co-gel (mixed 1:1 gel) of Keggin-ion complexed a-Fe2Os and TiCh NCs. Insets (clockwise from upper left in each panel) are photos of each gel, magnified views (all unlabeled size bars are 50 nm), and polyhedral-notation structures of the POM-ligated NCs;

[0023] Figs. 2A-2B illustrate the self-limiting assembly of negatively charged gel walls in the gelation process of the Li salt of an exemplary POM-NCs complex. Formation of percolated network of thin, negatively charged walls (Fig. 2A), encloses water-filled cavities, featuring a double layer of Li+ions near the negatively charged complexes (Fig. 2B);

[0024] Figs. 3A-3B demonstrate the dynamic movement and retention of Li+countercations during the process of PW11-SnO2gel formation. Fig. 3A is a collection of7Li NMR spectra taken at different stages of concentrating the complex solution in the gelation process (2.5%, 25% and 50% of criticalgelation concentration (CGC). Fig. 3B is a double-Lorentzian fitting of the broadened (" Gel") NMR spectra;

[0025] Fig. 4 shows the effect of photochemical reactivity of Li5[α-PW11O39]-complexed TiO2gel following exposure to UV light for 8 hours. The gel sample was suspended in a reactor containing 10% methanol solution stirred with a magnetic bar;

[0026] Figs. 5A-5B are bar graphs showing the turnover number (TON; Fig. 5A) and the turnover frequency (TOF; Fig. 5B) of H2 gas production by photochemical reduction of Li5[α-PW11O39]-complexed TiO2in its gel (" Gel") and non-gel (" Solution state") forms;

[0027] Fig. 6 is angle X-ray scattering (SAXS) spectrum showing data for a hexaniobate-complexed SnO2nanocrystals ([Nb6O19]-SnO2) gel. Inset: structure factor;

[0028] Fig. 7 is a high-resolution cryogenic transmission electron microscopy (cryo-TEM) tomography image of the face of the [Nb6O19]-SnO2gel wall (the same gel analyzed with SAXS shown in Fig. 7). Upper insets: side views;

[0029] Figs. 8A-8B are Cryo-SEM Images of a PWll-CeCh gel prepared from H+the protonated complex (H+form) and from the Na+bearing complex (Na+form). Size bars - 500 nm;

[0030] Figs. 9A-9C are Cryo-SEM Images of three "heterojunction" or co-gels formed by coassembling two different POM-complexed metal-oxide nanocrystals. Fig. 9A: hexaniobate-complexed CuO NCs (Nb6-CuO) co-assembled with hexaniobate-complexed SnO2NCs (Nb6-SnO2); Fig. 9B: heteropolytungstate-complexed FeCrO2NCs (PW11-FeCrO2) co-assembled with heteropolytungstate-complexed CeO2NCs (PWn-CeO2); Fig. 9C: PW11-α-Fe2O3co-assembled with PW11-TiO2;

[0031] Fig. 10 is bar graph showing the turnover number (TON) of H2gas production under UV–visible irradiation of PW11-ligated mixed α-Fe2O3 / TiO2co-gel and of aqueous solution of PW11-TiO2containing methanol; and

[0032] Figs. 11A-11B are SEM images of an aerogel prepared from PW11-TiO2gel using the carbon dioxide–based critical-point drying (CPD) method. Two different resolution scales are shown.DETAILED DESCRIPTION

[0033] Metal-oxide nanocrystals are widely used in colloidal, gel, and solid forms for catalytic, optical, electronic, and electrochemical applications. In known semi-solid systems, including colloidal dispersions and nanocrystal gels, stability is typically achieved by electrostatic surface charging or by organic ligand coatings. Although electrostatically stabilized colloids provide high surface reactivity, they are highly sensitive to pH, ionic strength, and concentration, and therefore readily aggregate or precipitate. Organic ligands enable control over nanocrystal size, shape, and crystal phase, but they block access to reactive surface sites and act as electronic insulators, limiting charge transfer, catalytic activity, and optoelectronic performance.

[0034] Solid and semi-solid nanocrystal assemblies are commonly produced by sol-gel processes, ligand stripping, ligand exchange, or destabilization-induced aggregation. Sol-gel methods involve irreversible chemical transformations and often yield amorphous or poorly controlled oxide structures. Ligand-removal and oxidative stripping approaches can expose nanocrystal surfaces but frequently result in loss of colloidal stability, uncontrolled aggregation, or limited compositional tunability. Gelation strategies based on irreversible destabilization can produce solvogels and aerogels; however, such systems generally lack reversibility and do not permit homogeneous postsynthetic functionalization, controlled cation exchange, or incorporation of multiple nanocrystal types into compositionally uniform heterojunction networks.

[0035] The present disclosure overcomes these limitations by employing inorganic polyoxometalate (POM) cluster anions as robust, organic-ligand-free ligands for metal-oxide, metal-hydroxide, and metal-oxyhydroxide nanocrystals. As previously demonstrated by the present inventors, POM ligands can bind strongly, including covalently, to nanocrystal surfaces, introducing new structural and electronic features (see, e.g., WO 2023 / 175512). POMs are polyanionic clusters, typically carrying charges from -3 to -8, and multiple POM ligands can be attached to a single nanocrystal, resulting in highly charged anionic complexes whose overall charge may range from approximately -20 to -200. These charges are balanced by countercations such as Na+, Li+or K+, imparting high solubility and dispersion stability.

[0036] Unlike conventional organic ligands, POM ligands bind to only a fraction of surface metal atoms, leaving a large number of reactive nanocrystal surface sites exposed to the surroundingmedium. At the same time, the negative charges of the POM ligands and their countercations stabilize the nanocrystals and regulate their electronic structure. As a result, POM-complexed nanocrystals exhibit enhanced stability, controlled surface reactivity, improved electron transfer, and tunable redox behavior. In addition, POMs can function as electron donors and / or acceptors, further enhancing catalytic, photochemical, and photocatalytic performance.

[0037] Based on these properties, the present inventors discovered that concentrated aqueous solutions of POM-complexed metal-oxide, hydroxide, or oxyhydroxide nanocrystals spontaneously and reversibly assemble into gels through a self-limiting process. Gelation arises from the formation of negatively charged gel walls that define a percolated three-dimensional network of solvent-filled nanocavities. Importantly, this assembly process is reversible and thermodynamically controlled, rather than being driven by irreversible chemical transformations.

[0038] The resulting POM-nanocrystal gels retain the crystallinity and high surface reactivity of the individual nanocrystals while providing exceptional mechanical and chemical stability. Unlike previously known nanocrystal gels, these materials permit homogeneous pre- and post-synthetic functionalization, including reversible exchange of countercations with transition-metal ions, incorporation of multiple nanocrystal types to form heterojunction gels, and controlled conversion into high-surface-area aerogels. Accordingly, the disclosed POM-nanocrystal gels and derived aerogels provide a versatile and previously unavailable platform for chemical, electrochemical, photochemical, and photoelectrochemical applications.

[0039] POM-nanocrystal gels

[0040] In one aspect, the present disclosure relates to gels comprising one or more polyoxometalate-complexed nanocrystals of Formula (I):

[0041] Q / [ / -POM]mH / [NC].

[0042] The indicator m is the number of POM ligands bound to an individual nanocrystal (NC). m may be any number, including fractions, between 1 to 50,000, for example, between 1 to 10,000 or 1 to 6000.i is the structural isomer of POM. It may be absent if POM has no isomers.

[0043] is the number of amounts of H+associated with the complex. reflects the number of H+that counterbalance the negative charge of the complex and is equal or less than the overall negative charge of all POMs in the complex. If each POM bares a net negative charge n-, then / < n m. The complex is said to be fully protonated if f- n m, and partially protonated if / < n.

[0044] In some embodiments, m is any number from 2 to 1000.

[0045] The POM itself is typically a negatively charged ligand, also referred to herein as "cluster-anion". The negative charge on the ligand is at least partially counterbalanced by Q, which is one or more cations selected from H+, an inorganic cation, or an organic cation such as, but not limited to, an ammonium-type cation of the formula RXH4-XN+, wherein R is an alkyl (e.g., n-alkyl) or thioalkyl group and x is an integer from 1 to 4.

[0046] An inorganic cation, as referred to herein, is a positively charged ion that does not contain an organic (carbon-based) framework, and is typically derived from an element or simple inorganic compound. Inorganic cations commonly include metal ions (such as alkali metals, alkaline earth metals, transition metals, and lanthanides) as well as simple non-metal cations.

[0047] Examples of inorganic cations include alkali and alkaline earth metal cations such as Li+, Na+, K+, Cs+, Mg2+, Ca2+; transition-metal cations such as Fe2+ / Fe3+, Co2+, Ni2+, Cu2+; and lanthanide cations such as Ce3+, Eu3+, Gd3+. In the context of polyoxometalates and nanocrystal complexes, inorganic cations function as countercations that balance negative charge, mediate electrostatic interactions, and influence assembly, stability, and reactivity of the material.

[0048] In some embodiments, Q is an alkali metal cation such as Li+, K+, Na+or Cs+. The index I denotes the relative number of Qs, which counterbalance the charge of the POM, and is an integer of 1 to 75. In organic-solvent-soluble complexes, at least some of the Qs are organic cations, for example, n-Bu4N+and / or n-Octyl4N+In some protonated POM complexes disclosed herein, Q is H+.

[0049] Polyoxometalates (POMs) typically contain transition-metal atoms from group 6 of the periodic table, such as molybdenum (Mo) or tungsten (W), or less commonly from group 5, including vanadium (V), niobium (Nb), or tantalum (Ta), generally in high oxidation states. A POM cluster is composed of metal-oxygen building units, denoted herein as MOt, where M is the POM's metal ion and b represents M's coordination number. In most cases, b - 6, although coordination numbers of4, 5, 7, or higher can also occur. Herein M represents one, two, three, four or more types of metal atoms, i.e., with respect to its metal ions, a POM can be homogeneous (comprising one type of ionized metal atoms) or heterogeneous (comprising two or more types of ionized metal atoms).

[0050] The fundamental building units of POMs MOt are polyhedral metal-oxygen units, most commonly MO6octahedra in which the metal center is coordinated by six oxygen atoms in an approximately octahedral (pseudo-octahedral) geometry. These octahedra assemble by sharing corners and / or edges (via shared oxygen atoms) to form a wide variety of cage-like cluster structures. The most stable POM structures are formed through corner- and edge-sharing of MO6units, which spatially separate the positively charged metal ions and thereby reduce electrostatic repulsion. The coordination environment of the oxygen atoms depends on their position within the cluster: surface oxygen atoms are typically terminal or doubly bridging (two coordinated) oxo ligands, while internal oxygen atoms are commonly triply bridging or, in some cases, coordinated to multiple metal centers (e.g., even six coordinated). A complete POM framework may be generally represented as MpOy, wherein p and y are the relative amounts of metal and oxygen atoms, respectively.

[0051] The metal atom M in a polyoxometalate (POM) is also referred to herein, interchangeably, as a secondary, peripheral, or addenda atom. The addenda atom M may comprise a single metal species or a combination of two or more different metal species. A POM cluster anion containing more than one type of addenda metal is referred to herein as a mixed-addenda cluster.

[0052] In general, M comprises at least one metal cation, including one, two, three, or more transition metal and / or main-group metal cations, optionally in a high oxidation state. As used herein, transition metals refer to metallic elements from Groups 3-12 of the periodic table, characterized by partially filled d-orbitals and the ability to adopt multiple oxidation states. Representative transition metals include scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), hafnium (Hf), tantalum (Ta), tungsten (W), and silver (Ag).

[0053] Polyoxometalates are most commonly constructed from early transition metals, which, as used herein, refer to transition metals located toward the left side of the transition series (typicallyGroups 3-6), that readily form high oxidation states and stable metal-oxygen frameworks. Exemplary early transition metals include titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W). These metals, particularly in high oxidation states such as V(V), Nb(V), Ta(V), Mo(VI), and W(VI), are especially well suited for forming robust POM frameworks with favorable redox and catalytic properties.

[0054] In some embodiments, M comprises one or more early transition metal cations selected from V(V), V(IV), Nb(V), Ta(V), Mo(V), Mo(VI), and / or W(VI). In further embodiments, M comprises any combination of two, three, four, or more transition metal cations, preferably in higher oxidation states.

[0055] In addition to transition metals, main-group metals capable of existing in high oxidation states may also serve as addenda atoms M. Main-group metals, as used herein, refer to metallic elements from Groups 1, 2, and 13-15 of the periodic table and include, without limitation, lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), aluminum (Al), gallium (Ga), indium (In), tin (Sn), lead (Pb), and bismuth (Bi). In particular, p-block heavy metals (post-transition metals) that can exist in higher oxidation states include: (i) Group 13 metals such as Al(lll), Ga(lll), In(lll), and Tl(lll); (ii) Group 14 metals such as Sn(IV) and Pb(IV); and (iii) Group 15 metals such as Bi(lll) and Bi(V).

[0056] Such main-group metals may be incorporated into POM frameworks and can modify the electronic, redox, and catalytic properties of the resulting clusters. In some embodiments, M comprises one or more of Al(lll), Ga(lll), In(lll), Tl(lll), Sn(IV), Pb(IV), Bi(lll), and / or Bi(V).

[0057] In addition to metal atoms M and oxygen atoms O, a polyoxometalate (POM) framework may further include one or more additional elements, designated herein as X, which may also be referred to as a primary heteroatom, core heteroatom, central heteroatom, or heterocation. The element X is typically four- or six-coordinate and is positioned at or near the center of the POM framework. The identity of X is not particularly limited and may include hydrogen, one or more positively charged heteroatoms, and / or one or more positively charged metal cations.

[0058] In some embodiments, X comprises one or more non-metal heteroatoms selected from phosphorus (P), sulfur (S), and fluorine (F), and / or one or more metalloid heteroatoms selected from arsenic (As), antimony (Sb), silicon (Si), and germanium (Ge). In some embodiments, X may compriseone or more protons (H+), including two protons acting collectively as the central heteroatom. In other embodiments, X comprises one or more positively charged metal ions selected from main-group metals, transition metals, lanthanides, or combinations thereof.

[0059] Lanthanide elements, while not typically forming classical POM frameworks on their own, may be incorporated into hybrid POM structures. In such embodiments, lanthanide ions (Ln3+), including but not limited to La3+, Ce3+, Nd3+, and Eu3+, may substitute for one or more metal centers within the POM framework or act as countercations associated with negatively charged POM clusters, forming lanthanide-POM coordination complexes. Lanthanides may also bridge multiple POM units forming lanthanide-bridged clusters (such high-nuclearity POM clusters are useful in magnetic and catalytic applications) or coordinate to framework oxygen atoms, thereby modifying (e.g., improving) the electronic, catalytic, magnetic, or optical properties of the resulting POM structures. Exemplary lanthanide-containing POMs include lanthanide-substituted Keggin-type and Wells-Dawson-type POMs.

[0060] In certain embodiments, X comprises one, two, three, or more metal cations selected from Be, Na, Al, Ga, Ti, Zr, Hf, V, Cu, Fe, Mn, Co, Sn, Pb, Ce, or combinations thereof.

[0061] Although there are no strict limitations on the identity of X, representative examples include non-metal heteroatoms such as phosphate or silicate groups, as well as positively charged metal ions in various oxidation states, including Fe(II / III), Co(I / II), Ni(II / IV), and Zn(ll), and combinations thereof.

[0062] In general, X may be hydrogen, one or more positively charged heteroatoms, and / or one or more positively charged metal atoms, wherein the metal atoms are selected from main-group metals, transition metals, lanthanides, or combinations thereof. In certain embodiments, X comprises one or more heterocations of a non-metal element selected from phosphorus (P), sulfur (S), and fluorine (F), and / or one or more heterocations of a metalloid selected from arsenic (As), antimony (Sb), silicon (Si), and germanium (Ge).

[0063] Based on the presence or absence of a central heteroatom X, POMs may be structurally classified into two general categories. Isopolyanions (IPAs), also referred to as iso-polyoxometalates, have the general formula [MpOy]n−and lack a central heteroatom. Heteropolyanions (HPAs), also referred to as heteropolyoxometalates, include one or more heteroatoms X and have the generalformula [XzMpOy]n“, where z < p. Although this classification is useful, many POM structures exhibit additional complexity due to incorporation of multiple addenda metals and / or multiple heteroatoms.

[0064] Recurring structural motifs define the major POM families. IPAs typically consist of octahedrally coordinated metal centers, while HPAs form distinct architectures determined by the coordination geometry of the central heteroatom. Representative HPA motifs include the Keggin, Anderson, and Wells-Dawson structures, which differ in heteroatom coordination number and overall topology.

[0065] The Keggin structure refers to ot-Keggin anions having the general formula [XM12040]n“, wherein X is a heterocation (e.g., P5+, Si4+, B3+), and M is an addenda metal such as molybdenum (Mo), tungsten (W), vanadium (V), titanium (Ti) or combinations thereof, e.g., W and Ti. The structure self-assembles in acidic aqueous solution and is notable for its chemical and redox stability in catalytic reactions at suitable pH values. Structurally, the Keggin anion comprises a central [Xm'O4](8’mtetrahedron (m' is the positive oxidation state of X. The total charge of four oxygen ions is -8, thus, the overall charge of the tetrahedron depends on how positive X is), surrounded by twelve pseudo-octahedral MO6units linked through shared oxygen atoms (a total of 24 bridging oxygen atoms link the 12 addenda atoms). The twelve addenda metals are arranged as four M3O13 triads, almost equidistant from each other, giving rise to overall tetrahedral symmetry. The Keggin framework may be reversibly hydrated, dehydrated, reduced, and oxidized without significant structural change. Five structural isomers exist, designated a, (3, y, 6, and s, which arise from different rotational arrangements of the M3O13units, with the ot-isomer generally being the most stable.

[0066] In the Anderson structure, represented by [XM6024]n“, the heteroatom X is octahedrally coordinated by six oxide ligands. In contrast, the Wells-Dawson structure, [X2M18062]n“, contains two tetrahedrally coordinated heteroatoms. Wells-Dawson derivatives containing a metal vacancy, such as [X2M17062]n“, provide defined metal-binding sites, the location of which is designated as α1or α2depending on the position of the defect within the structure.The Lindqvist structure, an example of an IPA, has the general formula [M6O19]n−and consists solely of metal and oxygen atoms. The central oxygen atom in this structure is distinct from the terminal oxides, as it is coordinated to all six metal centers.

[0067] (POMs with p-block elements (X = P, Si, Al, Ga, Ge...), transition metal elements (X = Fe(II / III), Co(I / II), Ni(II / IV), Zn(ll)...), and even two H+have been synthesized.

[0068] POM frameworks are assembled through bridging ligands, which are atoms or groups of atoms that connect two or more metal ions. Such bridging ligands are designated by the symbol μ, with a subscript indicating the number of metal centers bridged. In POMs, bridging ligands are typically oxygen atoms (ju-oxo ligands), which may bridge two metals (μ2-O) or multiple metals (e.g., μ3-O or μ4-O). Hydroxo ligands (μ-OH) may also serve as bridging groups. In the context of the present disclosure, bridging ligands may connect metal centers within the POM framework and / or metal centers of complexed or ligated nanocrystals.

[0069] Polyoxometalates generally contain metal-oxygen bonds of varying multiplicity and strength. For example, in a Keggin anion such as [PW12040]3-(i.e., X = P, M = W), each addenda metal is coordinated to one terminal oxo ligand, four doubly bridging μ2-O ligands, and one triply bridging μ3-O ligand that also connects to the central heteroatom.

[0070] In some embodiments, POM is presented by the Formula (II):

[0071] [XzMpDdOy]n−

[0072] wherein the elements X, M and O are as defined supra.

[0073] D is selected from H, OH, H2O (also denoted herein as OH2), or an organic moiety, including a lower alkyl, lower hydroxyalkyl, lower silylalkyl, lower silylalkoxy, and / or a carboxylate group. Optionally, D is covalently linked directly to M and / or O of the POM framework. In some embodiments, D is OH, and the POM is referred to herein as a hydroxylated POM, represented as [XzMpOy(OH)d]n“, wherein d denotes the relative number of hydroxy groups.

[0074] The indicators z, p, d, and y represent the relative amounts of X, M, D, and O, respectively. In some embodiments, z is from 0 to 100, p is from 6 to 250, d is from 0 to 100, and y is from 15 to800. Each of these values may be an integer or a fractional value within the indicated ranges. The indicator n, representing the overall negative charge of the anion, is an integer from 1 to 75.

[0075] As used herein, the term "lower alkyl" refers to a saturated, branched or unbranched hydrocarbon group having from 1 to 6 carbon atoms (C1–C6alkyl), including but not limited to methyl, ethyl, n-propyl, / so-propyl, butyl, / so-butyl, sec-butyl, tert-butyl, n-pentyl, / so-pentyl, neopentyl, tert-pentyl, n-hexyl and / so-hexyl. In some embodiments, the lower alkyl is a C1–C3alkyl.

[0076] The terms "lower hydroxyalkyl" and "alkoxy", used interchangeably herein, refer to a lower alkyl substituted with one or two hydroxy groups, provided that two hydroxy groups are not attached to the same carbon atom. Representative examples include hydroxymethyl, hydroxyethyl, hydroxypropyl, dihydroxypropyl, and dihydroxybutyl groups. In some embodiments, the lower hydroxyalkyl is HO–C1, HO–C2or HO–C3alkyl.

[0077] The term "lower silylalkyl" refers to a group derived from silane (SiH4), wherein the silicon atom is covalently bonded to one, two, or three lower alkyl groups. When one or more of these substituents is an alkoxy group, the moiety is referred to herein as a silylalkoxy group.

[0078] The term "carboxylate", also referred to as a carboxylate ion, denotes the conjugate base (RCO2_) of a carboxylic acid (RCO2H), wherein R is hydrogen or a lower alkyl group as defined herein.

[0079] In some embodiments, D is absent (d = 0). In other embodiments, D comprises a C1–C3alkyl, a C1–C3hydroxyalkyl, or any combination thereof.

[0080] As used herein, the term "unit cell" refers to the smallest repeating structural unit of a crystalline material that, when repeated in three dimensions, forms the entire crystal lattice. The unit cell is defined by three edge lengths (a, b, c) and three interaxial angles (a, P, y). Common unit cell types include primitive (simple), where the atoms only at the corners; body-centered cubic (BCC), where there is additional atom at the center; face-centered cubic (FCC), where the atoms are at corners and center of each face; and base-centered unit cell, where the atoms are at corners and centers of two opposite faces. For example, cerium dioxide (CeO2) crystallizes in a face-centered cubic (FCC) unit cell, wherein cerium atoms occupy the corners of the unit cell and oxygen atoms occupy the face-centered positions.

[0081] The core nanocrystal (NC) of a disclosed protonated POM complex comprises metal oxide, metal hydroxide or metal oxyhydroxide unit cells, represented by Formulae (lll)-(V), respectively:[M'iOk]; (III)

[0082] [M'iOHt] (IV)

[0083] [M'iOk(iOHt)] (V)

[0084] wherein i denotes the relative amounts of metal M', whereas k and t denote the relative amounts of oxygen O and hydroxyl (OH), respectively. Each of / , k and t may independently assume values from 0 to 20, including integer or fractional values such as 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, or 8. As used herein, the terms "relative amount" and "relative quantity" are interchangeable and refer to the proportion, ratio, or abundance of a given element relative to other elements in the empirical chemical formula of the complex.

[0085] In a broad sense, M' represents one or more metal cations selected from main-group metals, transition metals, and / or lanthanide elements, which in combination with oxide, hydroxide, or oxyhydroxide form nanocrystals that may contain oxygen vacancies.

[0086] As used herein, an "oxygen vacancy" is a point defect in a metal oxide structure resulting from the absence of an oxygen atom at a crystallographic lattice position, which may be charge-compensated by changes in the oxidation state of one or more neighboring metal ions. The vacant site can enhance electrical conductivity, chemical reactivity, or catalytic activity.

[0087] Main-group metals include elements from the s-block and p-block of the periodic table. Representative main-group metal oxides that may form oxygen-deficient nanocrystals include oxides of magnesium (MgO), calcium (CaO), strontium (SrO), barium (BaO), aluminum (AI2O3), gallium (Ga2O3), indium (I n2O3), tin (SnO2), lead (PbO / PbO2), and bismuth (Bi2O3). Oxygen vacancies in such oxides can enhance reactivity and are useful in catalysis, environmental remediation, and energy-related applications.

[0088] Certain alkali metals such as lithium (Li), rubidium (Rb), and cesium (Cs) can form oxides exhibiting oxygen vacancies (e.g., Li2O, Rb2O, Cs2O); however, these oxides are unstable in water and rapidly convert to hydroxides, which are highly soluble and therefore generally unsuitable for forming stable POM-complexed hydroxide nanocrystals.

[0089] In some embodiments, M' is a main-group metal cation selected from Mg, Ca, Al, Ga, In, Sn, Bi, or combinations thereof.Transition metals (Groups 3-12) readily form oxide nanocrystals exhibiting oxygen vacancies that enhance electronic, catalytic, optical, and magnetic properties. Representative transition-metal oxide nanocrystals include, without limitation, oxygen-deficient TiO2-x, V2O5-X, Cr2O3-x, MnO2-x, Fe2O3-x / Fe3O4, Co3O4-x, NiO_x, CuO_x, ZnO_x, MoO3-x, and WO3-X. Wherein x is the number of vacancies. Such nanocrystals are widely used in photocatalysis, energy storage, sensing, and redox catalysis.

[0090] Lanthanides (elements 57-71, also known as rare-earth elements (REEs)), characterized by their partially filled 4f orbitals, form oxides that can exhibit oxygen vacancies and possess unique redox, optical, and magnetic properties. A well-known example is cerium dioxide (CeO2-x), which exhibits reversible oxygen storage (reversible reduction and oxidation), for example by reduction followed by re-oxidation with dioxygen (O2), preferably under ambient conditions using air, therefore this metal oxide nanocrystal is widely used in catalysis.

[0091] Other lanthanide oxides suitable for forming oxygen-deficient nanocrystals include La2O3-x, Pr6O11 / Pr2O3-x, Nd2O3-x, Sm2O3-x, Eu2O3-x, Gd2O3-x, Tb2O3-x, and Dy2O3-x.

[0092] In some embodiments, M' is a lanthanide cation selected from Ce3+, La3+, Nd3+, Eu3+, Tb3+, Dy3+, Gd3+, or combinations thereof.

[0093] Lanthanides readily complex with POMs due to their strong Lewis acidity, large ionic radii, and affinity for oxygen-rich ligands. Eu3+, Tb3+, Dy3+and Gd3+that have optical and magnetic properties, readily complex with POMs forming complexes that can enhance redox activity, charge transfer, luminescence, magnetism, and catalytic performance.

[0094] The core nanocrystal may comprise two or more different metal cations (selected from main-group, transition metal and / or lanthanides), forming binary or ternary nanocrystals. Nonlimiting examples include BaTiO3, SrTiO3, KNbO3, LiNbO3, and LiTaO3. In some embodiments, one metal cation acts as a dopant and is present in a smaller amount relative to the other metal cations to modify electronic or catalytic properties.

[0095] Further embodiments include transition-metal-doped or mixed-metal oxide nanocrystals such as transition-metal-doped e-MnO2, CeO2, SnO2, or TiO2.In some embodiments, the NC comprises metal oxide unit cells, [M'iOHt] (Formula (IV). Exemplary metal hydroxide NCs include In(OH)3, Co(OH)2, Ni(OH)2, Al(OH)3, Ga(OH)3, Sc(OH)2and others comprised of main group or transition-metal cations.

[0096] In some embodiments, the NC comprises metal oxy-hydroxide unit cells, [M'iOk(iOHt)] (Formula (V). With k contributing a charge of (-2) and t contributing a (-1) charge, these indicators sum to the total positive charge of the metal cations M', adjusted for relative abundance or occurrence in the empirical unit.

[0097] In some embodiments, the POM and / or nanocrystal core is protonated, namely, f = n. The number of unit cells in an NC may range from about 50 to about 100,000,000, depending on composition, symmetry, and dimensions.

[0098] In some embodiments, one or more metal cations represented by M' are shared between the nanocrystal and the POM ligand. In other embodiments, the metals of the nanocrystal remain confined to the NC core while being coordinated by the POM.

[0099] The present disclosure further encompasses defect or substituted POMs, also referred to as monovacant or vacant POMs, in which one or more addenda metals are absent or replaced. A monovacant POM contains a single vacancy that may be occupied by a different metal cation. Nonlimiting examples include monovacant Keggin- or Wells-Dawson-type POMs such as [α-XzW11O39](12−n)−and [α2-P2W17O61]10−.

[0100] In some embodiments, substituted POMs comprise ln(ll l)OH-substituted monovacant Wells-Dawson clusters, for example [α2-P2W17O61(In3+OH)]8−, which may bind to ln(OH)3nanocrystals through available hydroxo ligands.

[0101] POM-complexed nanocrystals disclosed herein may further assemble into amorphous or crystalline suprastructures comprising up to tens or hundreds of thousands of individual POM-NC complexes.

[0102] Non-limiting examples of POM-NC complexes include heteropolytungstate- or hexaniobate-complexed e-MnO2, SnO2, CeO2, ZrO2, HfO2, CuO, TiO2, Co(OH)2, BiVO4, Ni1.5NbO3H2, CrFeO3, γ-FeOOH (lepidocrocite), Fe5O3(OH)9(ferrihydrite), Al(OH)3, Ga(OH)3, Eu(OH)3and mixed-metal oxides such as Fe2O3 (hematite), with Cr(lll) as dopant and TiiSni′Ok, wherein i, i′ and k, are the relative amounts of Ti, Sn and O, respectively, and each independently may vary from 1 to 20. Therelative amounts of Ti and Sn may each independently range from about 5% to about 100% of the metal composition. For example, changing the relative amounts of Ti and Sn improves optical properties of the NCs. It also improves the charge separation and transfer, which is helpful for photocatalysis, advanced oxidation processes, and gas sensing.

[0103] As used herein, a heteropolytungstate refers to a tungsten-based polyoxometalate comprising W-0 framework units and at least one heteroatom incorporated into the cluster structure. An exemplary heteropolytungstate is [α-PW11O39] also abbreviated herein as PW11. a hexaniobate refers to a niobium-based polyoxometalate comprising a six-niobium oxide cluster, typically [Nb6O19], also abbreviated herein as Nb6. Complexation of such POM ligands with metal oxide, hydroxide or oxyhydroxide nanocrystals enables tuning of optical, electronic, and catalytic properties, including enhanced charge separation and charge transfer, which are advantageous for applications such as photocatalysis, advanced oxidation processes, and gas sensing.

[0104] In some embodiments, metal cations larger than Ti(IV) or Fe(lll) are coordinated to POM ligands in a tetra-coordinated "out-of-pocket" fashion rather than the more common pentacoordinated "in-pocket" mode. Such metal cations include, for example, Zr(IV), Hf(IV), Ag(I / III), Pd(II / IV), Pt(II / IV), Pb(II / IV), Cd(ll), and Ce(IV). Out-of-pocket metal cations may further bind to nanocrystal surface metal cations through one or more p-oxo bridges.

[0105] As used herein, the term "pentacoordinated in-pocket fashion" refers to a coordination mode in which a metal cation is bound within a vacancy or binding pocket of a polyoxometalate (POM) framework and is coordinated by five oxygen donor atoms belonging to the POM structure. In this coordination mode, the metal cation occupies a defect or substituted site in the POM framework, resulting in strong, thermodynamically stable binding due to multidentate coordination and close proximity to the POM core.

[0106] As used herein, the term "tetra-coordinated out-of-pocket fashion" refers to a coordination mode in which a metal cation is bound to a polyoxometalate (POM) ligand by coordination to four oxygen donor atoms located on the exterior of the POM framework, such that the metal cation remains positioned outside the central binding pocket of the POM cluster. In this coordination mode, the metal cation does not occupy a vacancy within the POM framework and is instead coordinated at the periphery of the POM, typically due to steric size or coordination preferences ofthe metal ion. The metal cation may further interact with one or more metal centers on a nanocrystal surface through bridging ligands such as μ-oxo or μ-hydroxo groups.

[0107] Gels disclosed herein can be assembled from either alkali-metal or H+countercation forms of the POM-NC complexes. For example, in some embodiments, both forms of countercation are employed in Keggin-heteropolytungstate-complexed NCs gels. Extensive dialysis of Keggin-derivative complexed NCs results in the replacement of alkali-metal countercations by H+at the surfaces of the metal-oxide NCs, transforming them into polyprotic cores for charge balancing the p-0 linked POM-anion ligands. Neutralization by addition of MOH (M = alkali-metal cation) fully restores the alkali-metal cation form. Throughout these exchange processes, the complexes remain highly soluble. As such, POM ligation liberates the metal-oxide NCs from iso-electric point precipitation, an inherent property of traditional charge-stabilized colloids that hinders the use of surface protonation to control reactivity. pH-independent solubility is also observed for hexaniobate-complexed NCs whose ligating Nb6O198−ions are polybasic, ranging from H6M2[Nb6O19] to M8[Nb6O19] (M = alkali-metal cation) with increase in pH from ca. 8 to >12.

[0108] Gels formed from POM-complexed NCs are also referred to herein, for brevity, as " POM-NC gels". Gels disclosed herein are formed by a self-limiting assembly of the ligated NCs. As solutions of POM-complexed NCs are concentrated forcing the particles closer together, crowding-induced repulsion is attenuated by countercation association with the negatively charged POMs on the nanocrystal surfaces. This counterion association partially neutralizes the surface charge and weakens the electrostatic repulsion between particles. With repulsion reduced, the nanocrystals can start to organize into early-stage (incipient), wall-like or network structures, rather than remaining fully dispersed.

[0109] As the nanoparticles (POM-complexed NCs) assemble into wall-like structures, small cavities filled with water form inside these walls. These tiny cavities trap positively charged ions (i.e., countercations), via an entropically favorable process. As a result, more cations accumulate inside the cavities, leaving the surrounding walls with an excess negative charge. As the walls thicken, the growing negative charge within the thickening walls causes electrostatic repulsion, and this repulsion eventually becomes strong enough to prevent or inhibit further additions of POM-complexed nanoparticles from attaching. At that point, wall growth naturally stops. This is why wallthickness is referred to herein as self-limited. Exemplary gels formed by concentration alone of freely diffusing water-soluble Keggin-ion complexed and hexaniobate-complexed metal oxide NCs are shown in Figs. 1A-1H.

[0110] The self-limiting assembly of negatively charged gel walls formed from POM-nanocrystal complexes is illustrated in Figs. 2A-2B using the lithium salt as an exemplary system. As the concentration of the negatively charged POM-nanocrystal complexes increases during solvent removal, electrostatic crowding between neighboring complexes develops. This inter-particle repulsion is initially mitigated by the electrostatically favorable migration of Li+countercations into the regions between adjacent complexes. As a result, the local ionic strength within the forming wall regions increases dramatically, reaching values on the order of ~10 M, as estimated from elemental analysis and cryo-SEM observations. Such ionic strengths are far higher than those of conventional electrolyte solutions and reflect the highly confined nature of the inter-complex regions.

[0111] During wall formation, water becomes trapped between the negatively charged complexes. Retention of this confined water is entropically favorable and driven by osmotic pressure, as it reduces both crowding and electrostatic repulsion by maintaining a finite separation between neighboring anionic complexes. At the same time, Li+countercations preferentially remain in the surrounding bulk water to maximize entropy (Fig. 2A; black arrow). This imbalance results in the developing gel walls retaining a net negative charge, even as countercations accumulate nearby, forming electrostatic double layers adjacent to the walls (Fig. 2B).

[0112] Because the walls remain partially negatively charged, further growth is increasingly opposed by electrostatic repulsion. This leads to an arrested, self-limiting assembly process in which wall thickness reaches a defined maximum rather than continuing to grow indefinitely. The outcome is a percolated network of thin, negatively charged walls that enclose water-filled cavities, as shown in Fig. 2B, rather than precipitation of charge-neutral solids. This self-limiting mechanism explains the formation of stable, porous gel structures with well-defined architecture.

[0113] Embodiments disclosed herein relate to a reactive new class of functionalizable metal-oxide NC gels. As reversible, equilibrium-controlled nanostructured materials, these assemblies representan entirely new class of functionalizable and chemically responsive metal-oxide gels, amenable to pre- or post-synthetic modification.

[0114] For example, in photocatalytic activity, gels disclosed herein are dramatically more reactive than solutions of the same NC complexes. The present investors discovered that upon gelation, molar rates (per NC) of photochemical H2 evolution by PWn-TiCh, increased by 80-fold relative to a notably reactive solutions of PWn-TiCh, by 1500-fold relative to commercial anatase, and were four times faster than the rate of visible-light H2 evolution by Pt on "black" TiCh.

[0115] In some embodiments, co-gels are disclosed, formed by the co-gelation of two or more POM-metal oxide NCs complexes. As used herein, the term "co-gel" refers to a gel structure formed by the simultaneous or cooperative gelation of two or more distinct components, wherein at least one component comprises POM-metal oxide NC complex, such that the components assemble together into a single, integrated three-dimensional network rather than forming separate gels. " Cogelation" means that the components assemble, not separately such that the resulting gel contains both components interwoven or integrated at the nanoscale. In the context of the present disclosure, co-gelation is also referred to as synthetic modification of the straightforward gelation of a single POM-NC complex.

[0116] Co-gel are highly efficient as functionalizable gels in catalytic processes. For example, a-Fe2O3and TiO2are both semiconductors, having different electronic properties. When they are in intimate contact within the same gel, they form a heterojunction. As referred to herein, "heterojunction" is an interface between two different semiconductors that promotes separation of light-generated electrons and holes (positive charges), reduces charge recombination, and improves photocatalytic efficiency.

[0117] Co-gels comprising two or more types of POM complexes of semiconductor metal oxide NCs, for example, PWii-a-Fe20s and PWn-TiO2, are referred to herein as "semiconductor-heterojunction gels" or simply as "heterojunction gels", implying that the semiconductor interfaces are distributed throughout the gel network, not just at a flat surface.

[0118] Heterojunction gels are capable of UV-Vis light driven H2evolution from water alone. Light excitation generates electrons and holes in the semiconductor network, however, because of the heterojunction, these charges are efficiently separated and transported, and the co-gel can producehydrogen gas directly from splitting water. No sacrificial agents (such as methanol) are required. Such co-gels feature efficient charge separation, sufficient catalytic activity and effective use of light energy.

[0119] Gels disclosed herein can be subjected to post synthesis modifications. In the context of the gels and co-gels described herein, "post-synthesis modification" refers to one or more treatments applied after gelation or co-gelation of POM-metal-oxide, metal-hydroxide or metal-oxyhydrixide nanocrystal complexes, including but not limited to ligand exchange, partial or complete ligand removal, ion exchange, solvent exchange, doping, surface functionalization, thermal treatment, or chemical activation, wherein the three-dimensional gel network is substantially retained while its chemical, electronic, catalytic, or mechanical properties are modified. In other words, postsynthesis modification means changing or fine-tuning the gel after it has already formed, rather than during its initial assembly. These modifications allow the properties of the gel, such as conductivity, catalytic activity, or optical behavior, to be adjusted without rebuilding the material from scratch. These and other modification strategies provide numerous options for using POM-ligated NCs in the rational design of functional multi-component semiconductor-NC gels and aerogels.

[0120] In some embodiments, post-synthetic modification of the disclosed POM-nanocrystal gels is achieved through reversible exchange of countercations with transition-metal ions after gel formation. In this process, the native countercations associated with the polyoxometalate-ligated nanocrystal network are replaced by transition-metal ions without disrupting the three-dimensional gel architecture. For example, redox-active vanadyl (VO2+) ions can be incorporated directly into the gel walls, imparting new electronic and catalytic functionality while preserving the original structure. The incorporation of such ions has been directly observed by the present inventors using cryogenic scanning electron microscopy and confocal laser microscopy, which together preserve gel morphology and spatially localize the exchanged species.

[0121] These post-synthetic exchange strategies enable precise tuning of the chemical, electronic, and catalytic properties of gels and co-gels without structural degradation. The exchange process is reversible and can be chemically controlled, for example through chelation with EDTA, allowingcatalytic components to be introduced, removed, and regenerated. As a result, the gels are recyclable and suitable for repeated catalytic use.

[0122] A wide range of functionalized nanocrystal gels can be prepared by exchanging transitionmetal cations such as Fe3+, Co2+, Cu2+, or Ni2+into POM-ligated frameworks, thereby creating catalytically active sites without altering the nanocrystal cores. In some embodiments, the exchanged ions act as isolated single-site catalysts within the gel walls, while in others they may be reduced in situ to form metal-zero nanoparticles that catalyze hydrogenation or redox reactions. This reversible, modular approach enables the deliberate design of complex, multicomponent semiconductor nanocrystal gels and aerogels with tailored and regenerable catalytic properties.

[0123] POM-NC aerogels

[0124] The gel materials disclosed herein may further be converted into aerogels, for example by supercritical drying, while retaining high surface area, interconnected nanocrystal networks, and embedded catalytic sites. These aerogels constitute a new class of reactive, multi-component catalytic solids derived directly from the corresponding gel systems and combine structural robustness with tunable chemical functionality resembling those of the gels from which they were formed.

[0125] Thus, in another aspect, the present disclosure relates to aerogels comprising one or more polyoxometalate-complexed nanocrystals of the Formula (I): Ql[-POM]mHf[NC], as defined herein.

[0126] As used herein, the term "aerogel" refers to a solid, highly porous material formed by removing the liquid phase from a gel— typically by supercritical drying or other non-collapsing drying techniques— such that the interconnected gel network is substantially preserved. The resulting aerogel is characterized by low bulk density and high surface area.

[0127] Although gel structures may otherwise rearrange or collapse over time, replacing water with ethanol kinetically traps the network, effectively fixing the structure and preventing further reorganization. The trapped gel can then be dried by critical point drying, which removes the liquid without inducing capillary forces that would collapse the network, thereby converting the gel into a stable aerogel while preserving its wall-like architecture.Aerogels formed from POM-complexed NCs are also referred to herein, for brevity, as " POM-NC aerogels".

[0128] Uses and Industrial Applicability

[0129] The polyoxometalate-nanocrystal (POM-NC) semi-solid and solid materials disclosed herein, including hydrogels, heterojunction gels and aerogels, provide a versatile and modular materials platform with broad industrial applicability. These materials combine high surface area, tunable redox activity, structural stability, and ligand-free access to active interfaces, enabling their use in catalysis, energy conversion, environmental remediation, biomedical applications, and advanced materials manufacturing.

[0130] In a further aspect, the present disclosure relates to methods for performing a chemical, electrochemical, photochemical, or photoelectrochemical process, comprising contacting, exposing or subjecting a substance with a gel and / or aerogel of the present disclosure, thereby inducing, catalyzing, promoting and / or modulating a chemical or physicochemical transformation of the substance.

[0131] Non-limiting uses and / or industrial applicability of gels and aerogels in accordance with the methods disclosed herein include:

[0132] (i) Catalysis and redox chemistry. The POM-NC gels and aerogels disclosed herein are particularly suitable for heterogeneous catalytic applications involving oxidation, reduction, and redox transformations. Notably, protonated POM-NC systems enable dioxygen activation under near-neutral pH conditions, eliminating the need for strongly acidic environments typically required for such reactions. This property reduces corrosion, toxicity, and operational costs, and expands applicability to sensitive chemical and biological settings.

[0133] The POM-NC gels and aerogels systems disclosed herein further enable efficient quenching and control of reactive oxygen species (ROS) without requiring strongly basic conditions. This capability allows safe and controlled redox chemistry in aqueous and mixed-phase environments. Reversible exchange of transition-metal cations and incorporation of zero-valent metal (M(0)) nanoparticles within gel or aerogel walls provide tunable catalytic active sites for reactions includingwater oxidation, hydrogen evolution, carbon dioxide reduction, aerobic oxidation of organic substrates, and other small-molecule transformations.

[0134] (ii) Photocatalysis and energy conversion. Heterojunction gels and aerogels comprising two or more different metal-oxide nanocrystals form homogeneously distributed semiconductorsemiconductor interfaces that enhance charge separation and charge transfer. These materials are useful for photocatalytic and photoelectrocatalytic energy conversion, including solar-driven water splitting and hydrogen production. The absence of organic surface ligands prevents surface passivation, improving quantum efficiency and long-term photostability. Aerogel forms are particularly advantageous for gas-phase and flow-reactor configurations due to their high porosity and accessible active sites.

[0135] (Hi) Electrocatalysis and electrochemical devices. The disclosed POM-NC gels and / or aerogels are suitable for integration into electrocatalytic systems and electrochemical devices. Transition-metal ions incorporated into gel or aerogel can be electrochemically reduced in situ to generate catalytically active species.

[0136] (iv) Environmental and remediation applications. Protonated POM-NC gels and aerogels exhibit enhanced stability and longevity in aqueous environments and are resistant to aggregation across a wide pH range. These features make them suitable for environmental remediation applications, including degradation of organic pollutants, water purification, and air treatment. The ability to activate molecular oxygen and generate ROS under mild conditions enables oxidative remediation processes without added oxidants, improving safety and sustainability.

[0137] (v) Biomedical and bioactive applications. Certain protonated POM-NC gels display redox activity and ROS modulation at or near physiological pH (~7). This enables applications in biomedical contexts where extreme pH conditions are unacceptable. The materials may exhibit enzymemimicking behavior, including superoxide dismutase-, catalase-, or peroxidase-like activity, supporting potential use in antimicrobial treatments, wound healing, oxidative stress regulation, and related therapeutic or diagnostic applications. Enhanced stability and controlled solubility of gel contribute to consistent performance and improved handling in biomedical environments.

[0138] (vi) Analytical, sensing, and diagnostic uses. POM-NC gels and aerogels incorporating noble metal nanoparticles enable analytical applications such as surface-enhanced Raman spectroscopy(SERS) and other spectroscopic techniques. The materials are suitable for in situ and operando studies of catalytic and charge-transfer processes. Their tunable composition and structural integrity further support use in sensors, optoelectronic components, and multifunctional coatings.

[0139] (vii) Advanced materials and manufacturing. The ability to convert hydrogels into aerogels without collapse of the underlying nanostructure enables scalable fabrication of lightweight, high-surface-area, and mechanically stable materials. These forms are suitable for industrial reactors, structured catalysts, energy devices, and solid-state systems. The modular design of the POM-NC platform allows customization of composition, redox behavior, and physical form to meet specific industrial requirements.

[0140] It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the present disclosure.

[0141] As used herein the term "about" refers to ± 10 %.

[0142] The terms "comprises", "comprising", "includes", "including", "having" and their conjugates mean "including but not limited to".

[0143] As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a compound" or "at least one compound" may include a plurality of compounds, including mixtures thereof.

[0144] Throughout this application, various embodiments described may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosure. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individualnumbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0145] It is appreciated that certain features of the present disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the present disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other embodiment described herein.

[0146] Various embodiments and aspects of the present disclosure as delineated hereinabove and as claimed in the claims section below find experimental support in the following examples.

[0147] EXAMPLES

[0148] Materials

[0149] Unless otherwise indicated, all reagents were of analytical or reagent grade and were used as received without further purification. Deionized water (18.2 MQ-cm resistivity) obtained from a Millipore® Direct-Q water purification system was used in all aqueous preparations, reactions, and cleaning procedures.

[0150] Polyoxometalates (POMs), including heteropolytungstates and hexaniobates, were synthesized according to established literature procedures. Representative examples include Na7[α-PW11O39]-12H2O (prepared as described in Haraguchi et al., Inorg. Chem. 2002, 33(6): 1015-1020. https: / / doi.org / 10.1021 / IC00084A008), K9[a-AIW11O39]-l3H2O, and K8[Nb6O19]-l4H2O. Where applicable, alkali-metal countercations were exchanged by standard ion-exchange techniques. The identity and purity of POM salts were confirmed by appropriate spectroscopic methods, including31P NMR,27AI NMR, FTIR, and Raman spectroscopy.

[0151] Metal-oxide and metal hydroxide nanocrystals complexed with POM ligands were prepared using reagent-grade metal precursors, including salts of cerium, manganese (e.g., KMnO4), tin, titanium, copper, iron, chromium, niobium, cobalt, europium and nickel. Exemplary reagents included ammonium cerium nitrate ((NI- hCefNOs, potassium permanganate, tin(IV) chloride ( SnCl4-5H2O), titanium alkoxides (e.g., titanium(IV) tetra-isopropoxide (TTIP)), copper(ll) salts, chromium(lll) salts (e.g., CrfNOsh-SHzO), niobium pentoxide (NbzOs), europium chloride (EuCI3) andcorresponding alkali hydroxides and halide salts. Organic solvents such as ethanol, isopropanol, acetonitrile, and diethyl ether were used as received.

[0152] For preparation, purification, and post-synthetic processing of POM-nanocrystal complexes and gels, regenerated-cellulose dialysis membranes (molecular-weight cutoff approximately 12,000-14,000 Da) were employed.

[0153] Additional reagents, including acids, bases, countercation salts, oxidants, reductants, and probe molecules used for catalytic or spectroscopic studies, were obtained from commercial suppliers and used without further purification unless otherwise specified.

[0154] Instruments and Methods

[0155] Inductively Coupled Plasma

[0156] Inductively coupled plasma (ICP) is an elemental analysis technique used to determine the identity and concentration of elements in a sample. In ICP analysis, a liquid sample is introduced as a fine aerosol into an argon plasma generated by radio-frequency excitation. The plasma, operating at temperatures of approximately 6,000-10,000 K, causes complete vaporization and atomization of the sample, followed by excitation and / or ionization of the constituent elements. The resulting atomic or ionic species generate measurable signals that are characteristic of their elemental composition.

[0157] ICP is coupled to a detection system, most commonly optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS). In ICP-OES, excited atoms emit light at element-specific wavelengths as they relax to lower energy states. The emitted light is detected by a spectrometer, allowing simultaneous identification and quantification of multiple elements with high sensitivity, typically down to parts-per-bi Ilion levels.

[0158] Elemental analyses described herein were performed using a Spectro Arcos FHM22 ICP-OES instrument (AMETEK®) equipped with a vertical plasma torch, and data were processed using Smart Analyzer Vision software. Samples were appropriately diluted to fall within the instrument's calibration range and were analyzed without further chemical treatment.Cryogenic transmission electron microscopy (cryo-TEM)

[0159] Cryogenic transmission electron microscopy (Cryo-TEM) is a high-resolution imaging technique used to examine nanoparticles and soft materials in a hydrated, near-native state. Unlike conventional TEM, which requires drying or embedding, Cryo-TEM preserves sample structure by rapid vitrification at cryogenic temperatures (~-196°C or lower), thereby preventing ice crystallization and structural collapse.

[0160] In Cryo-TEM, a thin film of a liquid sample is deposited onto an electron-microscopy grid and rapidly frozen, typically by plunging into liquid ethane cooled with liquid nitrogen. The vitrified sample is maintained at cryogenic temperatures during imaging and examined using a high-energy electron beam (typically 80-300 keV), producing two-dimensional projections with nanometer- to near-atomic-scale resolution. Image processing methods, including tomography, may be used to reconstruct three-dimensional structural information.

[0161] Cryo-TEM enables direct visualization of nanocrystal size, shape, aggregation state, internal structure, and ligand or surface-layer organization, and is particularly useful for resolving porous networks, gel walls, and nanocrystal assemblies without dehydration artifacts.

[0162] In the studies described herein, vitrified samples were prepared using an automated vitrification system. Data were collected on a transmission electron microscope (FEI Tecnai 12 G2) operated at 120 kV under low-dose conditions, and images were analyzed using standard digital microscopy software.

[0163] Scanning electron microscopy (SEM)

[0164] Scanning electron microscopy (SEM) is an imaging technique used to examine the surface structure and morphology of materials at high magnification. In SEM, a focused beam of electrons scans across the sample surface and interacts with it. The electron beam generates different signals when it interacts with a sample, most notably secondary electrons (SE) and backscattered electrons (BSE). Secondary electrons are low-energy electrons emitted from the near-surface region of the sample and are highly sensitive to surface features. They are mainly used to image surface topography, revealing fine details such as texture, pores, and edges. Backscattered electrons are high-energy electrons from the incident beam that are reflected by atomic nuclei. Their intensitydepends on atomic number, so regions with heavier elements appear brighter. BSE imaging therefore provides compositional contrast. Together, SE and BSE signals are collected to produce detailed images that reveal surface topography, texture, and compositional contrast with high spatial resolution and a large depth of field.

[0165] Cryogenic scanning electron microscopy (cryo-SEM)

[0166] Cryogenic scanning electron microscopy (cryo-SEM) is a structural imaging technique in which a specimen is examined in a scanning electron microscope while maintained at cryogenic temperatures, typically between about -140 °C and -180 °C. The electron-matter interactions are the same as in conventional SEM: a focused electron beam scans the specimen surface, producing secondary electrons that provide surface-topography information and backscattered electrons that provide contrast related to material density and atomic number. The distinguishing feature of cryo-SEM is that the specimen is kept frozen during imaging.

[0167] By immobilizing water, solvents, and other volatile components in the solid state, cryogenic conditions suppress diffusion, eliminate capillary forces associated with drying, and substantially reduce beam-induced structural rearrangement. As a result, hydrated, soft, or metastable materials, such as gels, hydrogels, and porous networks that contain large amounts of solvent, can be imaged in a morphology that closely reflects their native or processing state, rather than a collapsed or distorted dried structure.

[0168] Sample preparation typically involves mounting the specimen on a thermally conductive cryo-stub, followed by rapid freezing to preserve the structure prior to molecular diffusion, phase separation, or ice crystal growth. Freezing may be achieved by plunge-freezing into liquid nitrogen or nitrogen slush, or by contact with a precooled metal surface. Unlike cryo-TEM, complete vitrification is not required, as cryo-SEM primarily probes micrometer-scale morphology. The frozen sample is transferred into a cryogenic preparation chamber under vacuum or dry inert atmosphere to prevent frost contamination and maintained below the devitrification temperature throughout handling and imaging.

[0169] Imaging is performed with the specimen held on a cryogenic stage inside the SEM chamber. Low accelerating voltages and beam currents are typically employed to minimize beam-inducedheating, charging, and radiation damage. Cryo-SEM provides high-contrast morphological information, including particle size, pore structure, wall thickness, connectivity, and fracture features. While limited compositional analysis may be possible, cryo-SEM is primarily used as a high-fidelity method for structural characterization of hydrated and solvent-containing materials, enabling visualization of features that would otherwise be lost during drying or evaporation.

[0170] High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)

[0171] HAADF-STEM is an advanced electron-microscopy technique used to image materials at nanometer to atomic resolution with strong compositional contrast. In HAADF-STEM, a finely focused electron beam is scanned across an ultrathin sample, similar to SEM but within a transmission electron microscope. Electrons that are scattered to high angles are collected by an annular (ring-shaped) detector positioned around the beam path. These high-angle scattered electrons arise mainly from elastic interactions with atomic nuclei.

[0172] A key feature of HAADF-STEM is Z-contrast imaging: the image intensity scales approximately with the square of the atomic number (Z2). As a result, regions containing heavier elements (higher Z) appear brighter than regions containing lighter elements. This makes HAADF-STEM especially powerful inter alia for distinguishing different elements within a material and visualizing interfaces, dopants, and heterojunctions.

[0173] Electron Paramagnetic Resonance (EPR)

[0174] Electron paramagnetic resonance, also known as electron spin resonance (ESR), is a spectroscopic technique used to study materials with unpaired electrons. It is widely applied to analyze free radicals, transition metal complexes, and defects in solids.

[0175] EPR is based on the interaction of unpaired electrons with an external magnetic field. Electrons possess spin and behave as small magnetic dipoles. In most atoms, electrons are paired, and their magnetic effects cancel; however, species such as free radicals and transition-metal ions contain unpaired electrons and are therefore paramagnetic. When such a sample is placed in an external magnetic field, the electron spin states split into two distinct energy levels (Zeeman effect), corresponding to spin-up (ms= +1 / 2) and spin-down (ms= -1 / 2) states. Upon exposure to microwaveradiation, absorption occurs when the microwave energy matches the energy difference between these spin states, causing a transition between them. This resonance occurs at a characteristic magnetic field strength and is detected as an EPR signal. The resulting EPR spectrum provides information about the presence and quantity of unpaired electrons, as well as their local chemical environment.

[0176] Key parameters in ERP include the g-value (or g-factor) a dimensionless quantity, which reflects how the electron responds to the magnetic field and varies depending on the surrounding molecular structure, and hyperfine coupling (a-value), which arises from interactions between the unpaired electron and nearby magnetic nuclei (such as hydrogen, nitrogen, or phosphorus), resulting in characteristic signal splitting. When an unpaired electron interacts with a nucleus with spin / , the resonance peak splits into 2 +1 lines. The separation between these lines is the hyperfine coupling constant a, measured in gauss (G) or MHz. Hyperfine coupling provides information about the number and type of nearby nuclei, the distance between the electron and the nucleus and the structure of radicals and metal complexes.

[0177] The free electron has a g-value of 2.0023, whereas in real systems, the g-value deviates due to interactions with the surrounding chemical environment. Metals, radicals, and different materials have characteristic g-values that help identify them.

[0178] Confocal laser microscopy (CLM)

[0179] Confocal laser microscopy, also referred to as confocal laser scanning microscopy, is an optical imaging technique that uses a focused laser beam and a spatial pinhole to selectively collect light only from a thin, well-defined focal plane within a sample. Following laser focal illumination, emitted or reflected light passes through a pinhole aligned with the focal plane, which blocks out-of-focus signal. The beam is scanned across the sample to build a 2D image; sequential images at different depths (z-stack) are combined to form a 3D dataset. By rejecting out-of-focus light, CLM produces high-contrast images with improved depth resolution compared to conventional light microscopy, and it enables optical sectioning and 3D reconstruction of thick or translucent materials. CLM can operate in fluorescence mode (detecting fluorescent labels or intrinsic fluorescence) or reflection / scattering mode (useful for non-fluorescent structures).This technique is particularly well suited for the characterization of hydrated, soft, and porous materials as it permits non-destructive imaging of samples in their native, wet state.

[0180] In the context of gel characterization, CLM enables direct visualization of the internal gel architecture, including pore structure, wall thickness, connectivity, and spatial heterogeneity. The technique is especially valuable for assessing permeability and molecular accessibility, for example by monitoring the penetration and diffusion of fluorescent or colored probe molecules into gel walls and pore networks over time. CLM can further be used to map the spatial distribution of gel components, including POM ligands, nanocrystals, and exchanged metal ions, when these species exhibit intrinsic fluorescence, scattering contrast, or are labeled with suitable fluorophores. In addition, CLM is well suited for monitoring post-synthetic modifications such as cation exchange, co-gelation, or ligand substitution, by visualizing changes that propagate from the gel surface into the bulk.

[0181] When combined with electron microscopy techniques such as cryo-SEM or cryo-TEM, CLM provides complementary mesoscale structural and functional information, linking nanoscale organization to bulk gel properties while preserving the hydrated state of the material.

[0182] Small-angle X-ray scattering (SAXS)

[0183] Small-angle X-ray scattering is a nondestructive analytical technique used to characterize the size, shape, internal structure, and spatial organization of materials on the nanometer length scale In SAXS, a collimated beam of X-rays is directed at a sample, and the X-rays are elastically scattered by variations in electron density within the material. The scattered intensity is measured at very small angles relative to the incident beam, typically below a few degrees, corresponding to real-space structural features ranging from approximately 1 to several hundred nanometers. Because SAXS does not require crystallinity or long-range order, it is particularly well suited for studying disordered, soft, or heterogeneous systems such as nanocrystal networks and gel structures, and it can be performed on samples in solution, gel, or solid form under near-native conditions.

[0184] The SAXS scattering profile contains information about both the individual scattering objects and their spatial arrangement. This information is commonly described in terms of a form factor and a structure factor. The form factor reflects the size, shape, and internal electron densitydistribution of individual particles or building units. The structure factor describes how these particles are positioned relative to one another and captures correlations arising from interparticle interactions, aggregation, or network formation. Features in the structure factor, such as peaks or shoulders, correspond to characteristic interparticle distances or domain spacings, while changes in its intensity and shape provide insight into clustering, connectivity, and long-range organization within the material.

[0185] By analyzing the combined form-factor and structure-factor contributions, SAXS enables quantitative determination of particle dimensions, interparticle spacing, degree of aggregation, porosity, and hierarchical ordering. Because measurements can be carried out in situ and under conditions relevant to processing or use, SAXS is especially valuable for monitoring self-assembly, gelation, and structural evolution overtime. In polyoxometalate-nanocrystal complexes-based gels, SAXS provides direct insight into nanoscale organization within gel walls.

[0186] Carbon dioxide-based critical-point drying

[0187] Carbon dioxide-based critical-point drying (CPD) is a drying method applied for converting solvent-containing porous materials into dry solids while preserving their internal structure. In this method, the liquid within the material is first exchanged with a solvent that is miscible with liquid carbon dioxide, after which the solvent is replaced with liquid CO2under pressure. The system is then heated and pressurized above the critical point of CO2, where no distinction exists between liquid and gas, and the CO2is subsequently removed without forming a liquid-gas interface. By avoiding capillary forces associated with evaporation, CO2-based CPD prevents pore collapse and structural shrinkage, enabling the production of aerogels and other high-porosity materials that retain high surface area and structural integrity.

[0188] Synthesis of POM-complexed metal oxide and metal hydroxide nanocrystals

[0189] Usually, an aqueous solution of a salt of the desired metal was used as a precursor (also referred to herein as "precursor solution"), and the pH was adjusted forming a hydrated form of the metal ion. Afterward, the POM was introduced to the solution. The sample was heated hydrothermally in an autoclave to facilitate condensation of the hydrated metal into metal-oxidenanocrystals (NCs) and attachment of the POM ligands to the surface of the metal oxide nanocrystals. The reaction mixture was then separated from the reaction by-products. Using alkali-metal salt, the POM-complexed NCs were selectively and reversibly precipitated, and the soluble by-products were discarded using a centrifuge. The purified complexes were redissolved, and the solution was optionally concentrated using air evaporation or centrifuge.

[0190] For the preparation of POM-complexes of NCs comprising two or three different metal cations, two or more precursor solutions were prepared by separately dissolving the salt forms of the metals to be joined or mixed (or structurally combined) in the NCs in either water or water-miscible solvent. The pH of each solution was adjusted if needed. Then, the precursor solutions were combined, while vigorously stirring one solution and, optionally, dropwise adding the other solution. Afterward, POM was introduced, the sample was heated hydrothermally, and the reaction mixture was treated in a similar manner as described above for POM-complexed metal oxide NCs.

[0191] Importantly, the robust nature of the linkages between POM ligands and the complexed NCs were achieved via high-temperature NC formation simultaneously with ligation, which arrests NC growth. This unexpectedly strong coordination is critical to gel formation and for facilitating the selflimiting assembly of POM-complexed NCs.

[0192] In exemplary embodiments, three types of ligands (POMs) were used for forming soluble complexes: at neutral or slightly acidic pH, mono-defect Keggin or Wells-Dawson (WD) anions, which were penta- or tetra-dentate ligands for metal cations via "in"- or "out"-of-pocket coordination, were employed. A key point is that the POM-coordinated metal ions were bound via remarkably stable μ-O linkages to metal atoms at the NC surface. At basic pH values, hexaniobate anions, which are tri-dentate ligands, were used. While hexaniobate binding to NCs is less well understood, their highly electron-dense bridging-oxide ligands bind strongly to metal atoms at NC surfaces.

[0193] By way of example, the synthesis of hexaniobate-complexed europium hydroxide by the hydrothermal method is described. Hydrated europium chloride, (EuCl3·6H2O; 29.3 mg, 0.08 mmol) was dissolved into 8 mL of deionized water, and stirred for 10 minutes, giving a clear solution (pH ~5). Then, hexaniobate ligand, K8Nb6O19·16H2O (58.5 mg, 0.040 mmol) was added followed by the addition of freshly prepared 2 N NaOH (0.08 mL, 0.160 mmol) to raise the pH to ~12, resulting in a white turbid solution. A final volume of 10 mL was obtained by the addition of deionized water. Thewhite turbid solution was stirred (900 rpm) at room temperature for 24 hours to ensure complete hydrolysis of Eu (III) ions. Hexaniobate-complexed Eu(OH)3 NCs were obtained as a cloudy solution after 24 hours of hydrothermal reaction in a 23-mL Teflon-lined 316 stainless-steel reaction vessel at 180 °C.

[0194] For isolation and purification of the complex, the nano-sized hexaniobate-complexed Eu(OH)3was first precipitated by the addition of 1 M NaCI. Under these conditions, reversible aggregation of the POM complexed NCs decreased their solubility in water, enabling their separation from the supernatant solution by centrifugation (30 min at 6000 rpm). Notably, millimolar concentrations of the primary hexaniobate by products and unreacted EuCl3, were fully soluble in 1 M KCI and remained in the supernatant solution. After decanting the supernatant solution by pipette, a hydrated white gelatinous solid, was collected and dissolved in 10 mL of pure water, to give a clear, transparent solution. Two additional purification cycles of precipitation by the addition of NaCI, followed by centrifugation and re-dissolution in pure water, were carried out. Any remaining traces of byproducts still present in the solution, along with small amounts of NaCI precipitated out with the nanocrystals, were removed by dialysis for 48 h in pure water (1 L). This involved placing the solution in a cellulose membrane in a 1 L beaker, during which time, the water outside the dialysis membrane was replaced every 12 hours.

[0195] Linkages between POMS and metal-oxide NCs are remarkably robust, meaning that POM-complexed nanocrystals could be processed, transferred between solvents, purified repeatedly, and chemically modified without losing their functional surface chemistry. For example, otherwise inorganic, water-compatible heteropolytungstate complexes were made compatible with organic solvents by replacing their inorganic countercation with the organic countercation R4N+cations, (R - n-alkyl), thereby enabling transfer of the POM-complexed nanocrystals from water into organic solvents, without hampering the heteropolytungstate attachment to the nanocrystals and causing structural damage. Furthermore, hexaniobate-complexed NCs remained unchanged after repeated salting out and redissolution cycles, chemically stressful process that might break weak surface bonds. This stability of POM-NCs complexes was essential for gel and co-gel formation, postsynthesis modifications, device fabrication, catalytic and electronic applications.EXAMPLE 1

[0196] Synthesis of POM-complexed NC gels

[0197] POM-complexed NCs were typically prepared as solutions (100 mL) of approximate concentration (ca.) of 2-3 pM and purified by two cycles of saltingout using 1-2 M alkali-metal cation salts, centrifugation and redissolution in water. After a third salting out, the hydrated pellets were suspended in 10 mL of water, dialyzed against water until clear, and concentrated to ca. 0.1 mM by centrifugation in a diafiltration tube (45-60 min at 6000 rpm). The volume of the resultant clear solutions was then reduced by one half by gentle air flow targeted at the liquid surface. After which the vials were sealed and left overnight at ambient temperature.

[0198] Gel characterization utilized various methods, including cryo-SEM, high resolution-cryo-TEM tomography, and confocal laser microscopy (CLM), as described in Instruments and Methods. Exemplary gels are shown in Figs. 1A-1H.

[0199] Gel formation occurred whether the POM counter-cations were alkali-metal cations or H+, and whether the POM ligand was a Keggin heteropolytungstate anion, stable at pH values of ca. 3 to 7 (Figs. 1A-1E), or the hexaniobate anion, stable at pH values of from ca. 8 to above 10 (Figs. IF and 1G). For example, when the PW11-CeO2complex comprising Na+as the countercation (also referred to herein as " Na+form of PW11-CeO2") was used, gels formed just as readily as from the H+form of the same complex, using the same procedure described above. This is notable considering the dramatic effects of different countercations on the physical properties, reactivities and assembly of POM cluster-anions.

[0200] Moreover, gels were formed from: (1) uniformly small, ca. 1.5-2 nm NC core complexes (e.g., Figs. 1C and IF); (2) much larger, as well as polydisperse 10 ± 4 nm cores (Fig. 1A); (3) mixtures of Keggin-complexed NCs, e.g., 1:1 α-Fe2O3:TiO2(Fig. 1H) and of FeCrOs and CeO2; and (4) mixtures of hexaniobate-complexed NCs such as CuO and SnO2. This striking diversity and indifference to NC-size, dispersity, ligand type, pH and countercation, points to a common fundamental basis for gel formation, which is attributed to the self-limiting assembly of negatively charged gel walls. Notably, results provided herein represent the first extension of the fundamental principle of nanoscaleassembly to gelation.The cryo-SEM images in Figs. 1A-1H show the gel walls and the water filled nanocavities that percolate throughout the gel structure for various gels. These are believed to be the first cryo-SEM images of metal-oxide NC hydrogels.

[0201] EXAMPLE 2

[0202] Recovery of damaged gel

[0203] A Nb6-SnO2gel was subjected to dehydrative-damage by immersion of sealed vial containing the gel in a 90 °C bath (up to the height of the gel) until shrinkage and cracking occurred, followed by shaking the vial to obtain a two-phase system resulting from liberating condensed water from the upper edges of the vial. The vial containing the damaged gel was left at room temperature. Spontaneous "healing" occurred after 2 hours of equilibration, giving a smooth gel. A similar result was obtained for a PW11-CeO2gel (Na+salt).

[0204] These results showed that gel formation is determined by thermodynamic equilibrium, i.e., the gel structure depends only on the final conditions (such as concentration, pH, and ion composition) and not on how the system was prepared. As a result, the gel formed reproducibly and could be reversibly assembled by restoring the same conditions.

[0205] EXAMPLE 3

[0206] Partition of countercations between water-filled nanocavities and gel walls

[0207] In order to confirm that gel walls are negatively charged, the movement and retention of countercations were measured during the process of gel formation. In this study, lithium ions (Li+) were measured as tracers and studied with7Li NMR. The distribution of these countercations between water-filled nanocavities and gel walls was investigated using the Li+form of the diamagnetic complex PW11–SnO2during its gelation. The results shown in Figs. 3A-3B demonstrate that when the gel-forming components became more concentrated, the NMR signal changed, indicating that Li+ions were interacting with the forming gel.

[0208] The critical gelation concentration (CGC) is the minimum concentration of a material required to form a continuous, self-supporting gel under given conditions. Below the CGC, particles or molecules remain dispersed or form small aggregates. At or above the CGC, these componentsconnect into a three-dimensional network that spans the entire volume of the liquid, producing a gel. For gels formed from POM-nanocrystal complexes, the CGC is the complex concentration at which individual POM-NC units begin to percolate, forming interconnected walls and networks. This network formation is driven by electrostatic interactions, ion distribution, and entropy-related effects.

[0209] In the gelation process, as the concentration of POM-nanocrystal complexes increased to approximately 50% of the CGC, the7Li NMR signal progressively broadened, with a more pronounced change observed upon gel formation (Fig. 3A).

[0210] Double-Lorentzian fitting is a spectral analysis method in which a measured signal is modeled as the sum of two Lorentzian peaks, each representing a distinct population or environment of the same species. This approach is used when a single species exists in two distinct environments, and the signal from each environment overlaps in the spectrum. A Lorentzian peak is a mathematical function utilized herein for describing the shape of the7Li NMR signals for nuclei undergoing rapid motion or exchange. The peak is characterized by a center position (frequency), a width (related to mobility, interactions, or lifetime), and an intensity (related to how many species contribute).

[0211] In the context of the present study, one Lorentzian represents "free" Li+(mobile, in bulk water) whereas the other represents "bound" cations (associated with gel walls or confined regions). By fitting the spectrum with two Lorentzian components, one can determine the fraction of ions in each environment (from peak areas), differences in mobility or interaction strength (from peak widths), and evidence for rapid exchange between environments (broadened or merged peaks).

[0212] Analysis of the broadened signal using double-Lorentzian fitting was characteristic of quadropolar nuclei rapidly exchanging between free and bound sites and interpreted as rapid exchange of Li+ions between free and wall-associated environments (Fig. 3B). Quantitative integration showed that, of the total Li+concentration in the gel (approximately 150 mM), about 50% remained in a free state, while the remaining 50% was associated with gel walls (i.e., 75 mM located in the aq. cavities, or as double layers outside gel walls). Assuming that gel walls occupy approximately 5% of the total gel volume, the local Li+concentration within the wall regions was estimated to be about 1.5 M, corresponding to a roughly twenty-fold enrichment relative to thefree Li+population. This high concentration of positive ions within a small wall volume implies that the walls themselves must be strongly negatively charged. These results provide direct experimental evidence that gel growth stops naturally due to electrostatic repulsion, which is the basis of the selflimiting assembly mechanism.

[0213] EXAMPLE 4

[0214] Transition-metal cation exchange in Li4[α-PTiW11O39]3-complexed TiCh nanocrystal gel

[0215] Aqueous solutions of Mn(ll), Co(ll), Ni(ll) and Cu(ll) were made, with a concentration of at least 1 eq of the Li(l) cations present in the original gel. Stoichiometric considerations were taken under account. Data confirming whether or not the cation exchange was successful, and to what extent, was obtained using inductively coupled plasma optical emission spectroscopy (ICP-OES; see Materials and Methods). Further analysis was done with various methods, depending on the type of cation, and its specific properties which suited certain characterization methods.

[0216] Previous ICP results and analysis showed that while lithium is the most prominent / major cation in the PTiW11O40gel, protons are present in both dilute as well as gel forms of the NCs, despite relatively short periods of dialysis done during the purification. During the first cation exchange performed in the gel form, it was discovered that a 1 eq solution, taking into consideration the +5 charge of the NCs, managed to replace only the Li+cations, but not the protons present.

[0217] Preparation of a transition-metal cation-exchanged Keggin POM-complexed anatase TiO2gel sample for ICP analysis, required treatment with ethylenediaminetetraacetic acid (EDTA). EDTA in its alkaline form (Na+form) was added to chelate the transition-metal ions bound to the POM-anatase nanocrystals, thereby displacing them and allowing sodium ions to reoccupy the countercation sites. This chelation-driven exchange converted the gel into a sodium-counterion form that readily dissolved in the dilute aqueous solution used for ICP measurements. In the absence of EDTA, gels containing transition-metal countercations (e.g., Mn2+or Co2+) tended to precipitate rather than dissolve, preventing proper sample handling and accurate ICP analysis.

[0218] Electron paramagnetic resonance (EPR) measurements were performed on gels comprising Mn-PTiW11O40and Cu-PTiW11O40by placing small gel samples in capillary tubes. Because manganeseand copper ions have unpaired electrons, they produce EPR signals. The EPR spectra obtained revealed how these metal ions are bound and arranged within the gel structure. Specifically, the spectra provided information about the symmetry of the metal ion's local environment, how the ion interacts with the POM and nanocrystal surface, and whether the coordination environment differs between the gel state and the corresponding solid-state form.

[0219] (i) Cation Exchange with Mn(ll). An aqueous solution of Mn(ll) cations was prepared by dissolving MnCl2salt in water. The Mn2+concentration was set to equal 1 eq of the concentration of counter-cations to be replaced in the NC complexes. One mL of the MnCl2solutions was dripped over the gel, and then removed after 24h. Throughout the exchange, the gel's color changed, from a light, opaque gray, to a mustard-like yellow. The change of color began on the surface of the gel and diffused downwards until coloring the entire gel. The gel was then washed with 1 mL of H2O three times to remove any excess cation present.

[0220] After the cation exchange process was completed (as qualitatively assessed by a stable color transformation of the entire gel), a sample of the yellow gel was treated with Na+-EDTA and subjected to ICP for quantitative analysis. The results are depicted Table 1:

[0221] Table 1. Elemental analysis via ICP of Li+, Ti and Mn2+in PTiW11O40gel, before and after countercation exchange

[0222] Li+cone. [mM] Ti cone. [mM] Mn2+cone. [mM]

[0223] Li-PTiW11O40gel 110 1360 0

[0224] Mn-PTiW11O40gel 0 1350 53

[0225]

[0226] ICP analysis demonstrated a quantitative (100%) stoichiometric exchange of Li+cations with Mn2+ions. These shows not only that Mn2+ions successfully replaced all Li+cations present in the nanocrystal gel, but also that the Mn2+ions selectively exchanged only the Li+countercations without displacing protons inherently present in the gel. This indicates that the exchange process specifically targets externally introduced cations, rather than protons generated in situ from water within the gel.A gel sample was further analyzed by electron paramagnetic resonance (EPR) to investigate the coordination environment and symmetry of Mn2+cations incorporated into the gel. This analysis was prompted by an observed color change of the Mn2+ions from their characteristic pale pink appearance in aqueous octahedral coordination to a yellow coloration upon interaction with the Keggin polyoxometalate-TiO2nanocrystal gel. It was hypothesized that this color change reflects a reduction in symmetry of the Mn2+coordination environment relative to the typical octahedral geometry.

[0227] The EPR spectrum exhibited signals characteristic of Mn2+in an octahedral environment, together with an additional "forbidden" transition appearing at lower field, indicative of symmetry distortion of the Mn2+ions (results not shown). While the presence of this forbidden signal confirms a deviation from ideal octahedral symmetry, the precise nature of the distortion could not be conclusively determined. This limitation is attributed to the high concentration of Mn2+ions in the gel, which leads to significant Mn-Mn interactions that complicate spectral interpretation. Upon drying the gel to a solid powder, the forbidden signal became more pronounced.

[0228] (ii) Cation exchange with Co(ll) and Ni(ll). An aqueous solution of Co(ll) and Ni(ll) cations was prepared by dissolving Co(NO3)2and Ni(NO3)2salts, respectively, in water. The Co2+and Ni2+concentrations were set to match one equivalent of the countercations to be replaced in the complexes so as to provide a stoichiometric one-to-one exchange. One milliliter of each metal salt solution was applied dropwise onto separate gel samples and allowed to remain in contact for 24 hours, after which the solutions were removed. During the ion-exchange process, a visible color change was observed in the gels, consistent with incorporation of the respective metal cations. Specifically, the gel exposed to the Co2+solution changed from a light, opaque gray to a pale pinkish-red color, while the gel exposed to the Ni2+solution developed a pale mint-green coloration. In both cases, the color first appeared at the gel surface in contact with the solution and gradually diffused inward until the entire gel was uniformly colored. Each gel was subsequently washed three times with 1 mL of water to remove excess, unbound cations.

[0229] Elemental analysis of the washed gels by ICP revealed partial countercation exchange, with approximately 10% of the Li+countercations replaced by either Co2+or Ni2+. Most of the lithium cations therefore remained unexchanged, and, consistent with observations for Mn2+-exchangedgels, no proton exchange was detected. The relatively low degree of exchange may reflect insufficient cation concentration under these conditions or a lower affinity of Co2+and Ni2+for the polyoxometalate framework compared to Li+.

[0230] To improve exchange efficiency, new Co(NO3)2and Ni(NO3)2solutions were prepared at threefold higher concentrations. Use of these solutions increased the extent of cation exchange to approximately 40%. These results suggest that Co2+and Ni2+bind less readily to the polyoxometalate complexes than Mn2+or Li+.

[0231] The formation of gels containing mixed countercations ("hybrid" gels) could exhibit modified catalytic or photochemical properties, including potential selectivity in gas production under ultraviolet or visible light irradiation.

[0232] (iii) Cation exchange with Cu(ll). An aqueous Cu(ll) solution was prepared by dissolving Cu(NO3)2in water, with the Cu2+concentration adjusted to approximately 1.5 equivalents relative to the total concentration of the Li+countercations. One milliliter of the Cu(NO3)2solution was applied dropwise onto a gel sample and allowed to react for 24 hours, after which the solution was removed. During the exchange process, the gel underwent a distinct color change from a light, opaque gray to a bright turquoise blue, characteristic of Cu2+incorporation. The color initially appeared at the gel surface in contact with the copper solution and progressively diffused throughout the gel until uniform coloration was achieved. The gel was subsequently washed three times with 1 mL of water to remove excess, unbound cations.

[0233] Initial assessment of cation exchange was performed qualitatively based on the complete color transformation from the gray coloration associated with Li+countercations to the turquoise hue characteristic of Cu2+. To obtain quantitative confirmation, a sample of the Cu2+-exchanged gel was analyzed by ICP. ICP sample was prepared using EDTA adjusted to pH 7, in which pH Na+concentration is reduced and proton availability in increased. Under these conditions, the gel dissolved readily and completely, enabling accurate ICP analysis. ICP results indicated complete (100%) stoichiometric exchange of Li+with Cu2+. Thus, Cu2+ions effectively replaced the Li+countercations in the gel. Similar to the previously observed Mn2+exchange, Cu2+did not displace protons present within the gel. These protons are presumed to originate from water associated withthe gel network. The consistent inability of exchanged cations to replace these protons suggests a potentially unique and stable proton association within the Keggin-anatase TiO2gel framework.

[0234] A sample of the Cu2+-exchanged gel was analyzed by electron paramagnetic resonance (EPR) spectroscopy. Given that EPR is well suited for the analysis of paramagnetic species such as Cu2+, this technique was employed to further characterize the gel and to allow comparison with previously studied Mn2+-containing gels, particularly to determine whether Cu2+incorporation produced symmetry distortions similar to those observed for Mn2+. The EPR spectrum of the Cu2+-containing gel exhibited features consistent with the typical coordination symmetry of Cu2+, with no evidence of significant distortion detected under the conditions examined (results not shown), contrary to the distortion observed in the Mn2+spectrum.

[0235] EXAMPLE 5

[0236] Photochemical reactivity of [α-PW11O39]-TiO2gel

[0237] Titanium dioxide (TiO2) is a metal-oxide semiconductor widely used because of its high chemical stability, low toxicity, and strong photocata lytic activity. TiO2absorbs ultraviolet light, generating excited electrons and holes that drive surface-mediated oxidation and reduction reactions. These properties make TiO2suitable for a wide range of applications, including photocatalysis, self-cleaning and functional surfaces, environmental remediation, and energy-related technologies. TiO2exists in several crystalline forms, primarily anatase, rutile, and brookite, of which the anatase polymorph is particularly important. Anatase TiO2typically exhibits higher photocatalytic activity than rutile due to its favorable electronic band structure, higher density of reactive surface sites, and more efficient generation and separation of photoinduced electron-hole pairs. As a result, anatase TiO2is especially effective in light-induced surface phenomena such as photoinduced hydrophilicity, where exposure to light renders the surface highly water-attractive through changes in surface chemistry and charge. Owing to these advantages, TiO2is regarded as a "green" catalyst and has been extensively investigated for hydrogen generation from water and advanced oxidation processes (AOPs) for the degradation of organic pollutants. Continued development of improved TiO2-based photocatalytic systems remains of significant interest, e.g., for water remediation and sustainable energy applications.Photochemical reactivity of Li5[α-PW11O39]-TiO2gel. Following formation, structural characterization, and quantification of [α-PW11O39]-complexed TiO2gel, its photochemical reactivity was evaluated through a series of ultraviolet (UV) irradiation experiments focused on hydrogen production. Experimental parameters were systematically varied to assess their influence on both preservation of the gel structure and photocata lytic activity. These parameters included gel placement and agitation, choice of countercation, solution composition, atmospheric conditions, and irradiation time.

[0238] Lithium was selected as the initial countercation for photochemical studies, as the lithium-containing gel most closely resembled previously reported non-gel POM-complexed nanocrystal systems

[0239] To facilitate direct comparison with previously reported photocatalytic studies of TiO2and POM-modified nanocrystal systems, photochemical experiments were conducted in a 10% methanol-water solution, with methanol serving as a sacrificial electron donor to suppress charge recombination and promote hydrogen evolution. Methanol rapidly scavenges photogenerated holes on TiO2, suppressing electron-hole recombination. This allows photogenerated electrons to accumulate and drive proton reduction to hydrogen. This same role of methanol has been documented extensively in TiO2, POM-TiO2, and other metal-oxide NC systems, and employing the same medium allows direct comparison of hydrogen evolution rates, turnover numbers (TON), and turnover frequencies (TOF) between the present POM-complexed gel system and earlier nanocrystal systems.

[0240] Lithium perchlorate (1 M LiClO4) was added to the reaction medium to provide an excess of lithium ions, thereby stabilizing the gel structure by minimizing leaching of lithium countercations from the POM-TiO2nanocrystals. The reaction vessels comprising Li5[α-PW11O39]-TiO2in its gel form or non-gel, [α-PW11O39]5-TiO2solution state were purged with argon prior to irradiation to establish an inert atmosphere and eliminate interference from dissolved oxygen during photochemical measurements.

[0241] During the photocatalytic experiment, the amount of reaction products formed under UV light, specifically gaseous products such as hydrogen, was measured over different irradiation periods. The reaction was carried out in a sealed reactor that contains both the liquid reactionmixture and a gas-filled space above it, known as the headspace. Before UV irradiation, the gas composition in the headspace was measured to establish a baseline (i.e., to confirm that no hydrogen or other reaction gases were present initially). After exposing the system to ultraviolet light for a defined period of time, the headspace was analyzed again (using gas chromatography) to determine how much hydrogen had been produced during irradiation.

[0242] Based on literature reports identifying approximately four hours of UV irradiation as a representative benchmark for TiO2photocatalytic performance, this time point was selected for primary comparison. Additional measurements were conducted after 8 and 24 hours to assess hydrogen evolution over extended irradiation periods.

[0243] The high nanocrystal concentration, large accessible surface area, and interconnected gel architecture of the POM-complexed metal oxide gels contributed to efficient photochemical activity. Upon UV irradiation, TiO2generates photoexcited electrons, which are transferred through the gel network and reduce protons present in the system to molecular hydrogen. This process can be represented schematically as: TiO2+ hν → e-+ h+; followed by proton reduction, 2e-+ 2H+→ H2(g).

[0244] Reduction of the titanium from Ti(IV) to Ti(lll) following UV exposure under inert conditions resulted in change of gel color to dark blue color, as shown in Fig. 4.

[0245] These measurements enabled calculation of the TON, defined as the number of moles of H2produced per mole of TiO2, as well as the turnover frequency (TOF), defined as the TON normalized per hour of UV irradiation. The TON and TOF of [α-PW11O39]-complexed TiO2in the gel and noncomplexed, solution state, are shown in Figs. 5A-5B.

[0246] EXAMPLE 6

[0247] POM ligand type-dependent gel-wall structure

[0248] The internal structure of gel walls formed by POM- NC complexes was examined using smallangle X-ray scattering (SAXS) and high-resolution cryogenic transmission electron microscopy (cryo-TEM) tomography (see Instruments and Methods). These complementary techniques were used to probe how different POM ligands influence nanoscale organization and connectivity within the gel network. The results are show in Figs. 6 and 7.For gels formed from hexaniobate-complexed SnO2nanocrystals ([Nb6O19]-SnO2or Nb6-SnO2), SAXS measurements revealed distinct inhomogeneities in the structure factor, indicating the presence of clustered nanocrystal domains separated by an average distance of approximately 3.6 nm (Fig. 6). The structure factor describes how nanocrystals are spatially arranged relative to one another. If particles are uniformly dispersed, the structure factor would be smooth and featureless. However, if particles form clusters, networks, or repeating spacings, the structure factor shows distinct features (peaks or shoulders). The structure factor obtained for Nb6-SnO2gel implies that nanocrystals were not randomly distributed but instead organized into domains or clusters with a preferred spacing between them. When the gel was directly imaged using cryo-TEM, it was seen to be composed of nanocrystal-rich walls, separated from each other by nanoscale voids of same size, namely, about 3.6 nm (Fig. 7).

[0249] The fact that two independent techniques, SAXS and Cryo-TEM, gave the same characteristic length scale strongly confirms that the gel walls have a real, well-defined nanoscale architecture whereby the nanocrystals are organized into a porous network (not randomly aggregated).

[0250] Furthermore, these findings suggest that adjacent nanocrystals in Nb6-SnO2gel are connected through direct ion-pairing interactions between hexaniobate ligands bound to different NC surfaces. Such ligand-ligand interactions are analogous to coordination-polymer-type structures previously reported for hexaniobate ligands on gold nanoparticles.

[0251] This mode of connectivity has important structural and functional implications. Upon conversion of these gels into aerogels, the preserved porous and layered architecture enables access to a rationally tunable class of catalytic materials based on polyoxoniobate chemistry. The spacing, connectivity, and chemical environment of the gel walls can be systematically adjusted, offering control over catalytic accessibility and reactivity.

[0252] In contrast, direct contact ion pairing is not expected for gels formed from Keggin-type heteropolytungstate-complexed nanocrystals. The larger size, different charge distribution, and coordination geometry of Keggin anions are anticipated to prevent the same type of close ligandligand association observed for hexaniobates.EXAMPLE 7

[0253] Preliminary assessments of physical and chemical properties of POM-NC gels

[0254] (i) Reversibility of gel formation

[0255] Reversibility of gel formation was demonstrated using PW11O39-FeCrO3gel (Li+as countercation). After dissolution of the gel, individual particles of the PW11-FeCrO3complex maintained their original sizes in water (results not shown).

[0256] (ii) Counteraction dependency of gel structure

[0257] Cation-dependent differences in gel structure were observed. Cryo-SEM measurements of gels prepared from H+and Na+forms of the PWn-CeCh complex revealed that wall thicknesses in gels of the H+form were ca. 50% narrower than those in gels prepared from the Na+form of the complex. Results are shown in Figs. 8A-8B. This countercation dependency of wall thickness is attributed to more extensive H+dissociation, giving more negatively charged and hence narrower walls.

[0258] (iii) Dependency of gel formation and solubility on the countercation

[0259] In 30% iso-propanol, gel of PW11-CeO2complex were obtained from the protonated complex (H+form), but not from its Na+form. This is explained by the high solubility of H+in the mixed organic-inorganic solvent as opposed to Na+.

[0260] In addition, PWn-CeO2gels were shown to be formable as mixed salts containing combinations of alkali metal cations (e.g., Na+) and organic ammonium cations (R4N+, where R is methyl, ethyl, or n-butyl), and to be stable in mixed alcohol-water solvent systems (ROH:H2O).

[0261] The ability to form gels with different types of countercations, including organic ammonium ions, and to operate in mixed water-alcohol environments, makes these materials highly tunable. By changing the countercations or solvent conditions, the chemical environment inside the gel can be adjusted to favor specific reactions.(iv) Reversibility of cation exchange

[0262] Reversible cation exchange was demonstrated by the uptake of Fe3+ions by a PW11O39-SnO2gel initially present in its sodium (Na+) countercation form at pH 5, resulting in the formation of an iron-exchanged gel. Subsequent treatment of the Fe3+-containing gel with sodium ethylenediaminetetraacetate (Na2H2EDTA), a strong chelating agent, selectively removed the Fe3+ions and restored the original Na+-countercation, thereby confirming the reversibility of the cation exchange process in the gels.

[0263] The ability of PW11-SnO2gels to reversibly bind and release transition-metal cations demonstrates that the cations are not permanently incorporated into the nanocrystal lattice but are instead electrostatically and / or coordinatively associated with the POM ligands. The POM-nanocrystal framework remained structurally intact during cation exchange and removal. The use of EDTA shows that transition-metal binding is chemically addressable and controllable.

[0264] POM-NC gels disclosed herein allow post-synthetic modification of gel composition without degrading the gel architecture. Since the gel can be reset to its original state it can provide recyclable or regenerable reactive systems. This reversibility is particularly important for catalysis, where different metal cations can be introduced or removed to tune activity. For example, with Fe3+being particularly relevant in coordination chemistry and redox-active materials, Fe3+-substituted PW11-SnO2gel can be applicable in redox and applications. Because the cation exchange is reversible and chemically controllable, the same gel framework can be repeatedly functionalized with different catalytic metal ions such as Fe3+, Co2+or Cu2+.

[0265] (v) Organic molecule permeability of POM-NC gels

[0266] Penetration of organic molecules into the interior gel-wall regions was demonstrated using confocal laser microscopy (CLM). Specifically, a colorless gel of Nb6-SnO2nanocrystal complex was treated with the dye methyl orange, and CLM imaging confirmed that the dye diffused into and was retained within the gel walls rather than remaining only on the external surface. This proved that small organic molecules such as organic reactants can actually enter the gel walls, where catalytic reactions take place. Hence, these gels can be optimized for selective aerobic oxidation of organicmolecules (important in green chemistry and catalysis), and biomedical applications, such as wound healing, where controlled production of reactive oxygen species is beneficial.

[0267] (vi) Metal(O) nanoparticle-accommodating functionalized gels

[0268] The ability of gels to accommodate metal nanoparticles such as copper, silver, or gold in their metallic (zero-valent) (M(0)) was assessed.

[0269] M(0) metals were incorporated into the walls of POM-complexed nanocrystal gels using two routes: (1) reducing metal ions inside the gel (in situ); and (2) Mixing pre-made metal nanoparticles into the gel as it formed so that they become trapped and stabilized within the gel walls.

[0270] Chemical reduction of exchanged Cu2+ions with hydrazine, maintained PWn-SnO2gel, structurally intact, indicating that formation of Cu(0) nanoparticles does not disrupt the gel architecture.

[0271] Confocal laser microscopy (CLM, 405 nm excitation) confirmed the presence of Ag(0) nanoparticles within gel walls following co-gelation of hexaniobate-stabilized Ag(0) nanoparticles (approximately 20 wt%) with Nb6-SnO2complex. Similarly, treatment of an Nb6-SnO2gel with HAuCl4resulted in incorporation of gold species within the gel walls, likely forming hydrolyzed Au(lll) and / or reduced Au(0) domains.

[0272] These experiments showing that the gel remained intact strengthen the notion that the material can be modified after it is made. The POM ligands play a key role here as they act like protective coatings that keep the metal nanoparticles from clumping together, preserving their activity.

[0273] Preliminary electrochemical data further indicated that exchanged Cu2+ions distributed throughout the gel were electrochemically reduced to Cu(0) by applying an electrical potential. When the gel is coated with Nafion, a proton-conducting, ion-permeable polymer, and placed in contact with a carbon electrode, the Nafion layer allows ionic transport while maintaining electrical contact. Upon applying a reducing potential at the carbon electrode, electrons are transferred from the electrode into the gel, where they reduce Cu2+ions to Cu°. Because the Cu2+ions are distributed throughout the gel rather than localized at the surface, this electrochemical reduction can occurthroughout the gel volume, leading to the formation of metallic copper species embedded within the gel walls. This demonstrates that the gel is electrically accessible and can support electrochemical transformations of incorporated metal ions, which is important for various applications, such as, but not limited to:

[0274] (i) catalysis: metallic copper, silver, and gold are active for many chemical reactions, including oxidation and small molecule transformations;

[0275] (ii) electrocatalysis: metal ions inside the gel can be converted into metal nanoparticles using electricity, allowing precise control over catalytic activity;

[0276] (iii) analytical measurements: silver and gold nanoparticles enhance Raman signals, enabling sensitive surface-enhanced Raman spectroscopy (SERS) of molecules bound within the gel; and / or (iv) scalable reactors: the gels can be dried into aerogels, creating solid materials with very high surface area, mechanically stable catalytic materials suitable for continuous gas-flow reactor applications.

[0277] (vii) In situ formation of hexaniobate-complexed metal hydroxide nanocrystals

[0278] Hexaniobate-complexed metal hydroxide nanocrystals, including Co(OH)2, Ni(OH)2, and Rh(OH)3, were formed by in situ hydrolysis of exchanged transition-metal cations that were incorporated into gel structures. Such materials share structural and electronic features with polyniobate-supported metal-hydroxide photocatalysts and are suitable for oxidation, reduction, and photoelectrocatalytic reactions.

[0279] EXAMPLE 8

[0280] Metal-oxide heterojunction POM-NC gels

[0281] Metal-oxide heterojunction gels (also referred to herein as co-gels) were formed by coassembling two or more different POM-complexed metal-oxide nanocrystals (NCs) into a single, continuous gel network. Gel wall structure was evaluated using cryo-SEM and confocal laser microscopy (CLM) measurements. Exemplary co-gels obtained are shown in Figs. 9A-9C.

[0282] Combined cryo-SEM and CLM demonstrated that green-colored hexaniobate-complexed CuO nanocrystals (Nb6-CuO, approximately 15 wt%) co-assembled with hexaniobate-complexedSnO2nanocrystals (Nb6-SnO2) to form mixed-oxide gel walls (Fig. 9A), indicating intimate nanoscale mixing of the two oxide phases within the gel architecture.

[0283] In experiment employing Keggin-type heteropolytungstate ligands, approximately 18.5 wt% of 1.5-nm PW11-FeCrO2nanocrystals were incorporated into the gel walls of 4-nm PW11-CeO2nanocrystals, forming a mixed-oxide heterojunction gel (Fig. 9B).

[0284] In another experiment, a gel composed of approximately 50 wt% 3-nm PW11-α-Fe2O3nanocrystals and 10-nm PW11-TiO2nanocrystals was formed, yielding a structurally continuous gel comprising multiple metal-oxide interfaces (Fig. 9C).

[0285] These heterojunction gels possessed exceptionally high internal surface area and inherently generated a very large number of heterojunction interfaces distributed throughout the gel walls. This architecture increases the density of semiconductor-semiconductor interfaces by orders of magnitude relative to conventional systems in which nanocrystals are deposited on planar semiconductor substrates. Importantly, the co-gels were formed without organic ligands, thereby preserving direct access to nanocrystal surfaces and eliminating surface passivation effects that typically inhibit charge transfer.

[0286] As a functional demonstration, the PW11-ligated mixed α-Fe2O3 / TiO2heterojunction gel generated molecular hydrogen under UV–visible irradiation using water as the sole reactant and did so at a higher rate than a corresponding solution of PW11-TiO2in the presence of methanol (see Fig.

[0287] 10).

[0288] During UV or visible-light irradiation, TiO2generates electron–hole pairs. The photogenerated electrons are responsible for reducing protons to form H2, while the holes drive oxidation reactions, including water oxidation, which produces O2. When oxygen is generated in the same system, it can act as a competing electron acceptor: O2can capture some of the photogenerated electrons, forming reduced oxygen species instead of allowing those electrons to reduce protons. As a result, not all photogenerated electrons contribute to hydrogen evolution. Therefore, the experimentally measured H2evolution rate reflects only the portion of electrons that successfully produce hydrogen and underestimates the intrinsic hydrogen-generation capability of the PW11–TiO2system.EXAMPLE 9

[0289] Preparation of POM-nanocrystal aerogels

[0290] Hydrogels comprising POM-complexed metal-oxide nanocrystals were converted into aerogels while preserving their porous, three-dimensional network structure. Hydrogels of PW11–CeO2and PW11-TiO2, and Nb6–SnO2, representing the two principal classes of POM ligands described herein (heteropolytungstate and hexaniobate, respectively), were transformed into aerogels using carbon dioxide–based critical-point drying (CPD). The detailed procedure described below is for the preparation of PW11-TiO2aerogel.

[0291] Preparation of aerogel from PW11-TiO2gel

[0292] Immediately before critical point drying, water in a 2 mL vial containing the PW11-TiO2hydrogel a was replaced by ethanol by adding 1 mL ethanol on top of the hydrogel. After 10 minutes, the liquid layer was removed. This process was carried out two more times, after which the gel was transferred to a Critical Point Dryer, Tousimis 931. Fifty milligrams (50 mg) of gel was placed into the sample holder, which was then filled with 30-40 mL of pure ethanol and thereafter purged by CO2 gas throughout the entire process. The dryer was then set to carry out critical drying, which consists of cooling heating cycles ranging from -1 to 43°C. After 40 minutes the system was vented and the aerogel was removed from the sample holder.

[0293] Immediate CPD following solvent exchange prevented capillary collapse of the gel network. Upon completion of the drying cycle, a monolithic aerogel was obtained. Two cryo-SEM images of the aerogel are shown in Figs 11A and 11B.

[0294] These results demonstrate that conversion of POM-nanocrystal hydrogels into aerogels retained interconnected networks, producing mechanically stable, high-surface-area solids, that are expected to preserve catalytic and electronic functionality of the metal oxide nanocrystals, making them suitable for catalysis and gas-flow reactors.

Claims

1. WHAT IS CLAIMED IS:

1. A gel comprising one or more polyoxometalate-complexed nanocrystals of Formula (I):Ql[-POM]mHf[NC],whereinPOM is a polyoxometalate anion bearing a net negative charge n-;Q is selected from H+, an inorganic cation, or an organic cation;NC is metal oxide, metal hydroxide or metal oxyhydroxide nanocrystal;I is the relative number of countercations Q, which counterbalance the charge n-of the POM;m is the number of POM ligands bound to an individual nanocrystal (NC);is 0 or equal or less than m multiplied by n (n m); and / is absent or a specific POM isomer.

2. The gel of claim 1, formed by equilibrium-controlled, self-limiting assembly mechanism in which the negatively charged POM-NC complexes organize into thin, percolated walls that enclose interconnected, water-filled nanocavities, thereby producing a continuous porous network.

3. The gel of claim 1 or 2, characterized by at last one of: (i) self-healing; (ii) providing reversible cation exchange; (iii) tunable by pre-synthetic and / or post-synthetic modifications that directly affect gel structure, chemical composition and / or reactivity.

4. An aerogel comprising one or more polyoxometalate-complexed nanocrystals of Formula (I):Ql[-POM]mHf[NC],whereinPOM is a polyoxometalate anion bearing a net negative charge n-;Q is selected from H+, an inorganic cation, or an organic cation;NC is metal oxide, metal hydroxide or metal oxyhydroxide nanocrystal;I is the relative number of countercations Q, which counterbalance the charge n-of the POM;m is the number of POM ligands bound to an individual nanocrystal (NC);is 0 or equal or less than m multiplied by n (n m); and / is absent or a specific POM isomer.

5. The aerogel of claim 4, having a wall structure corresponding to that of the gel from which it was formed.

6. The gel or aerogel claim 4 or 5, wherein m is any number between 1 to 50,000, optionally between 1 to 10,000, 1 to 6000 or 2 to 1000.

7. The gel or aerogel of any one of claims 1 to 6, wherein I is an integer of 1 to 75.

8. The gel or aerogel of any one of claims 1 to 7, wherein ≤ n·.

9. The gel or aerogel of any one of claims 1 to 8, wherein Q is at least one alkali metal cation selected from Li+, K+, Na+or Cs+.

10. The gel or aerogel of any one of claims 1 to 9, wherein Q is a mix of at least one alkali metal cation and at least one transition metal cation.

11. The gel or aerogel of any one of claims 1 to 10, wherein Q is a mix of at least one alkali metal cation, at least one transition metal cation and / or at least one organic cation, optionally wherein the organic cation is an ammonium-type cation RxH4-xN+, wherein R is an alkyl or thioalkyl group and x is an integer from 1 to 4.

12. The gel or aerogel of any one of claims 1 to 11, wherein the metal in the metal oxide, metal hydroxide or metal oxyhydroxide nanocrystal is a cation of an atom selected from Mg, Ca, Al, Ga, In, Sn, Bi, Ti, Ce, Gd, La, Li, Rb, Cs, Sr, Ba, Al, Si, Ge, Pb, Bi, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, La, Ce, Pr, W, Nd, Sm, Eu, Gs, Dy and / or Tb.

13. The gel or aerogel of any one of claims 1 to 12, wherein the metal oxide is selected from MgO, CaO, SrO, BaO, AI2O3, Ga2O3, ln2O3, SiO2, GeO2, SnO2, PbO, PbO2, Bi2O3, TiO2, TiO2, V2O5, Cr2O3, Fe3O4, Fe2O3, Co3O4, NiO, CuO, ZnO, MoO3La2O3, CeO2, Pr6O11, Pr2O3, WO3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Tb2O3, transition-metal doped epsilon-MnO2, transition-metal doped CeO2, or transition-metal doped SnO2, and any combination thereof.

14. The gel or aerogel of any one of claims 1 to 12, wherein the metal hydroxide is selected from Eu(OH)3, Fe5O3(OH)9, ln(OH)3, Co(OH)2, Ni(OH)2, AI(OH)3, Ga(OH)3Rh(OH)3, or SC(OH)2, and any combination thereof.

15. The gel or aerogel of any one of claims 1 to 14, wherein POM is selected from heteropolytungstate, hexaniobate, isopolytungstate, isopolyvanadate, isopolymolybdate, isopolyniobate, heteropolyvanadate, heteropolymolybdate, polyoxoniobate, and modified form thereof selected from defect POMs, substituted POMs and "out of pocket" metal binding POMs.

16. The gel or aerogel of claim 15, comprising two or more different POMs.

17. The gel or aerogel of any one of claims 1 to 16, comprising two or more different metal oxide or metal hydroxide nanocrystals.

18. The gel or aerogel of claim 17, in the form of heterojunction-gel or heterojunctionaerogel.

19. The gel or aerogel of claim 17 or 18, comprising a mix of hexaniobate-complexed CuO nanocrystals co-assembled with hexaniobate-complexed SnO2nanocrystals, heteropolytungstate complexed FeCrO2nanocrystals co-assembled with heteropolytungstate-complexed CeO2nanocrystals, or heteropolytungstate complexed FeCrO2nanocrystals co-assembled with heteropolytungstate-complexed with TiO2.

20. The gel or aerogel of any one of claims 1 to 19, comprising zero-valent metal (M(0)) nanoparticles.

21. The gel or aerogel of claim 10 or 20, wherein Q. and / or M(0) are incorporated by postsynthesis modification.

22. A method for performing a chemical, electrochemical, photochemical, or photoelectrochemical process, the method comprising contacting a substance with a gel and / or aerogel according to any one of claims 1 to 21, thereby inducing, catalyzing, promoting and / or modulating a chemical or physicochemical transformation of the substance.

23. The method of claim 22, wherein the substance comprises water, oxygen, hydrogen peroxide, carbon dioxide, nitrogen, an organic compound, or any combination thereof.

24. The method of claim 22 or 23, wherein the process comprises at least one of oxidation, reduction, redox cycling, oxygen activation, reactive oxygen species generation, reactive oxygen species quenching, hydrogen evolution, water oxidation and / or carbon dioxide conversion.

25. The method of any one of claims 22 to 24, wherein the process is carried out at nearneutral pH.

26. The method of any one of claims 22 to 25, wherein the process is selected from environmental remediation, energy conversion, chemical synthesis, sensing, or biomedical treatment.

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