Carbon Dioxide Capture by Niobium Polyoxometalate Fragmentation.

High oxidation state metal cations in the form of oxides, oxoanions, or oxoperoxoanions have diverse roles in carbon dioxide removal (direct air capture and point source). Features include providing basic oxygens for chemisorption reactions, direct binding of carbonate, and catalyzing low-temperature CO2 release to regenerate capture media. Moreover, metal oxides and aqueous metal-oxo species are stable in harsh, point-source conditions. Here, we demonstrate aqueous niobium polyoxometalate (POM) carbon capture ability, specifically [Nb6O19]8-, Nb6. Upon exposure of aqueous Nb6 to CO2, Nb6 fragments and binds chemisorbed carbonate, evidenced by crystallization of Nb-carbonate POMs including [Nb22O53(CO3)16]28-and [Nb10O25(CO3)6]12-. While Rb/Cs+ counter cations yield crystal structures to understand the chemisorption processes, K+ counter cations enable higher capture efficiency (based on CO3/Nb ratio), determined by CHN analysis and thermogravimetry-mass spectrometry of the isolated solids. Sum frequency generation spectroscopy also showed higher carbon capture efficiency of the K-Nb6 solutions at the air-water interface, while small-angle X-ray scattering (SAXS) provided insights into the role of the alkalis in influencing these processes. Tetramethylammonium counter cations, like K+, demonstrate high efficiency of carbonate chemisorption at the interface, but SAXS and Raman of the bulk showed a predominance of a Nb24-POM (HxNb24O72, x ∼ 9) that does not bind carbonate. Control experiments show that carbonate detected at the interface is Nb-bound, and the Nb-carbonate species are stabilized by alkalis, demonstrating their supporting role in aqueous Nb-POM CO2 chemisorption. Of fundamental importance, this study presents rare examples of directing POM speciation with a gas, instead of liquid phase acid or base.

Supramolecular Self-Assembly of Proteins Promoted by Hybrid Polyoxometalates.

Controlling the formation of supramolecular protein assemblies and endowing them with new properties that can lead to novel functional materials is an important but challenging task. In this work, a new hybrid polyoxometalate is designed to induce controlled intermolecular bridging between biotin-binding proteins. Such bridging interactions lead to the formation of supramolecular protein assemblies incorporating metal-oxo clusters that go from several nanometers in diameter up to the micron range. Insights into the self-assembly process and the nature of the resulting biohybrid materials are obtained by a combination of Small Angle X-ray Scattering (SAXS), Transmission Electron Microscopy (TEM), and Dynamic Light Scattering (DLS), along with fluorescence, UV-vis, and Circular Dichroism (CD) spectroscopy. The formation of hybrid supramolecular assemblies is determined to be driven by biotin binding to the protein and electrostatic interactions between the anionic metal-oxo cluster and the protein, both of which also influence the stability of the resulting assemblies. As a result, the rate of formation, size, and stability of the supramolecular assemblies can be tuned by controlling the electrostatic interactions between the cluster and the protein (e.g., through varying the ionic strength of the solution), thereby paving the way toward biomaterials with tunable assembly and disassembly properties.

Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry.

ConspectusPolyoxometalates (POMs, metals = V4/5+, Nb5+, Ta5+, Mo5/6+, and W5/6+) can be described as molecular metal oxides. The V, Mo, and W-POMs (classic POMs) exhibit rich structural diversity with interesting redox properties, acid catalysis, inorganic ligands, and colorimetric properties and behavior. Nb and Ta POMs, while structurally similar, are generally stable only in base and redox behavior is rare, and they are synthetically far less accessible. The V, Mo, and W-POMs have been studied for well over a century, Nb-POM chemistry has emerged in the last 20 years, and Ta-POM chemistry is yet to see consistent and significant advances. Early and current success in Nb-POM chemistry is owed mainly to hydrothermal synthesis, which is wholly unsatisfying, given the black box nature of this technique.For the last 5 years and as summarized in this Account, we have exploited decaniobate, [Nb10O28]6- (Nb10), as a foundation to perform room-temperature, nearly pH-neutral manipulations of Nb-POM solutions. Nb10, with a rare neutral self-buffering pH, responds to any interactions with electrolytes (specifically oxoanions and metal cations) by undergoing transformations, leading to new topologies. The ease of Nb10 transformation yielding new generations of Nb-POMs, akin to an inorganic analogue of biological model organisms such as the fruit fly, inspired the title of this Account. The common building unit born from the disassembly of Nb10 is [Nb7O20(OH, H2O)2](5-7)-, and the hydroxyl/aqua ligands provide reactivity for linking via condensation reactions, ligand exchange, heterometals, or oxoanions. We can coax these newly assembled Nb-POMs (detected by small-angle X-ray scattering, SAXS) to crystallize via the usual methods of vapor diffusion, salting out, and reduced temperature, and the single-crystal X-ray diffraction structures are valuable for understanding reaction mechanisms to fine-tune control and yield a landscape of topologies and compositions. Beyond providing an opportunity to comprehend and diversify POM chemistry, the reactivity of Nb10 yields highly soluble (i.e., >2 M Nb), nearly neutral aqueous solutions of niobium, ideal for the solution-phase deposition of thin films, demonstrated with LiNbO3, (Na,K)NbO3, Nb2O5, and heterometal-doped Nb2O5. The obtained films are cohesive and smooth, enabled by the tendency of these solutions to gel if simply evaporated quickly.Per our current endeavors, this gelation behavior provides an opportunity to develop new soft, flexible materials including inorganic networks, organic-inorganic networks, and porous solids and explore their material properties including base catalysis and sorption (i.e., CO2). Nb-POM (and Ta-POM) discovery and implementation of properties is far from complete. While heterometal (d and f element) substitution is easy with classic POMs, imparting a whole host of functions (tuned luminescence, catalysis, electroactivity, etc.), it remains a challenge with Nb-POMs due to pH incompatibility with most heterometals. This grand challenge that defies fundamental aqueous behavior of metal cations requires the creation of liquid mixtures that include polymer and/or ionic liquid components, and the creation of such reaction media can impact synthesis beyond POM chemistry. The goal of this Account is to describe the recent advances in Nb-POM chemistry, afforded by the Nb10 "fruit fly", and to also provide insight into the next large steps needed to advance Nb-POM chemistry.

Pertechnetate/perrhenate-capped Zr/Hf-Dihydroxide Dimers: Elucidating Zr-TcO4 Co-Mobility in the Nuclear Fuel Cycle.

Spent nuclear fuel contains heavy element fission products that must be separated for effective reprocessing for a safe and sustainable nuclear fuel cycle. 93 Zr and 99 Tc are high-yield fission products that co-transport in liquid-liquid extraction processes. Here we seek atomic-level information of this co-extraction process, as well as fundamental knowledge about ZrIV (and HfIV ) aqueous speciation in the presence of topology-directing ligands such as pertechnetate (TcO4 - ) and non-radioactive surrogate perrhenate (ReO4 - ). In this context, we show that the flat tetrameric oxyhydroxyl-cluster [MIV 4 (OH)8 (H2 O)16 ]8+ (and related polymers) is dissociated by perrhenate/pertechnetate to yield isostructural dimers, M2 (OH)2 (XO4 - )6 (H2 O)6 ⋅ 3H2 O (M=Zr/HfIV ; X=Re/TcVII ), elucidated by single-crystal X-ray diffraction. We used these model compounds to understand the pervasive 93 Zr-99 Tc coextraction with further speciation studies in water, nitric acid, and tetrabutylphosphate (TBP) -kerosene; where the latter two media are relevant to nuclear fuel reprocessing. SAXS (small angle X-ray scattering), compositional evaluation, and where experimentally feasible, ESI-MS (electrospray ionization mass spectrometry) showed that perrhenate/pertechnetate influence Zr/HfIV -speciation in water. In Zr-XO4 solvent extraction studies to simulate fuel reprocessing, we provide evidence that TcO4 - enhances extraction of ZrIV , and compositional analysis of the extracted metal-complexes (Zr-ReO4 study) is consistent with the crystallized ZrIV 2 (OH)2 (ReVII O4 - )6 (H2 O)6 ⋅dimer.

The Role of Alkalis in Orchestrating Uranyl-Peroxide Reactivity Leading to Direct Air Capture of Carbon Dioxide.

Spectator ions have known and emerging roles in aqueous metal-cation chemistry, respectively directing solubility, speciation, and reactivity. Here, we isolate and structurally characterize the last two metastable members of the alkali uranyl triperoxide series, the Rb+ and Cs+ salts (Cs-U1 and Rb-U1). We document their rapid solution polymerization via small-angle X-ray scattering, which is compared to the more stable Li+, Na+ and K+ analogues. To understand the role of the alkalis, we also quantify alkali-hydroxide promoted peroxide deprotonation and decomposition, which generally exhibits increasing reactivity with increasing alkali size. Cs-U1, the most unstable of the uranyl triperoxide monomers, undergoes ambient direct air capture of CO2 in the solid-state, converting to Cs4[UVIO2(CO3)3], evidenced by single-crystal X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. We have attempted to benchmark the evolution of Cs-U1 to uranyl tricarbonate, which involves a transient, unstable hygroscopic solid that contains predominantly pentavalent uranium, quantified by X-ray photoelectron spectroscopy. Powder X-ray diffraction suggests this intermediate state contains a hydrous derivative of CsUVO3, where the parent phase has been computationally predicted, but not yet synthesized.

Cerium Nanophases from Cerium Ammonium Nitrate.

Ceria nanomaterials with facile CeIII/IV redox behavior are used in sensing, catalytic, and therapeutic applications, where inclusion of CeIII has been correlated with reactivity. Understanding assembly pathways of CeO2 nanoparticles (NC-CeO2) in water has been challenged by "blind" synthesis, including rapid assembly/precipitation promoted by heat or strong base. Here, we identify a layered phase denoted Ce-I with a proposed formula CeIV(OH)3(NO3)·xH2O (x ≈ 2.5), obtained by adding electrolytes to aqueous cerium ammonium nitrate (CAN) to force precipitation. Ce-I represents intermediate hydrolysis species between dissolved CAN and NC-CeO2, where CAN is a commonly used CeIV compound that exhibits unusual aqueous and organic solubility. Ce-I features Ce-(OH)2-Ce units, representing the first step of hydrolysis toward NC-CeO2 formation, challenging prior assertions about CeIV hydrolysis. Structure/composition of poorly crystalline Ce-I was corroborated by a pair distribution function, Ce-L3 XAS (X-ray absorption spectroscopy), compositional analysis, and 17O nuclear magnetic resonance spectroscopy. Formation of Ce-I and its transformation to NC-CeO2 is documented in solution by small-angle X-ray scattering (SAXS) and in the solid-state by transmission electron microscopy (TEM) and powder X-ray diffraction. Morphologies identified by TEM support form factor models for SAXS analysis, evidencing the incipient assembly of Ce-I. Finally, two morphologies of NC-CeO2 are identified. Sequentially, spherical NC-CeO2 particles coexist with Ce-I, and asymmetric NC-CeO2 with up to 35% CeIII forms at the expense of Ce-I, suggesting direct replacement.

Plutonium and Cerium Perrhenate/Pertechnetate Coordination Polymers and Frameworks.

Spent nuclear fuel (SNF) contains transuranic and lanthanide species, which are sometimes recovered and repurposed. One particularly problematic fission product, 99TcO4-, hampers this recovery via coextraction with high valence metals, perhaps by complexation during aqueous reprocessing of SNF. There is limited molecular-level knowledge concerning the coordination chemistry between TcO4- or its well-known surrogate ReO4- and transuranic/lanthanide species. In the current study, we investigated the coordination of ReO4-/TcO4- with plutonium and cerium cations by structural and chemical characterization of a series of isolated extended solids. In this study, Ce represents both trivalent lanthanides and is considered a surrogate for Pu, respectively, in its common trivalent and tetravalent oxidation states. The structural elucidation of the seven isolated crystalline solids revealed that ReO4-/TcO4- directly connects to PuIV, PuVIO22+, CeIII, and CeIV in the terminal and bridging coordination modes, leading to 1-, 2-, and 3-dimensional frameworks. For example, ReO4- coordination to Pu(IV) formed a 1D chain or 2D framework, isostructural with previously isolated Th(IV) compounds. However, PuVIO22+ alternating with ReO4- led to a unique 1D chain, different from the prior-reported U(VI)/Np(VI)-ReO4-/TcO4- structures. Coordination of ReO4-/TcO4- with Ce(III) promotes the assembly of 3D frameworks. Finally, attempted synthesis of a Ce(IV)-ReO4- compound resulted in a 2D framework with a mixed-valence CeIII/IV. The highly acidic reaction conditions supported the reduction of both CeIV and TcVII, challenging isolation of compounds featuring these species. Only one TcO4-containing structure was obtained in this study (CeIII-TcO4 3D framework), vs the six total Ce/Pu-ReO4 compounds. Our three Pu-ReO4 crystal structures are the first reported and translated to atomic-level information about Pu-TcO4 coordination in nuclear fuel reprocessing scenarios, in addition to broadening our knowledge of bonding trends in the early, high-valence actinides.

Three Phases of Basic Zirconium and Hafnium Hydroxohalides.

Aqueous solutions of zirconium and hafnium (M) halides (X) with atomic ratios α = X/M near 1 form glasses on evaporation. Herein, we describe the preparation and properties of these glasses and discuss the nature of the crystal-glass equilibria beyond the pure glass compositions. Small- and wide-angle X-ray scattering (SWAXS) studies reveal increased polymerization as α decreases from 2 to 1. The glasses are found to be much denser than their crystalline counterparts. Crystals forming in contact with glasses retain the well-known Zr-tetrameric hydroxo cluster unit with hydroxide compensating for the lowered halide content. We find that the chemical formulas for all of the solid hydroxohalides may be described by the single parameter α, according to the formula M(OH)4-αXα·(4α - 1)H2O. This description is valid for the crystalline chloride (MOX2·8H2O = M(OH)2X2·7H2O), the glassy solids with α < 2, and hydrolyzed products (α ≈ 0.5). The water content is also determined by α with hydroxide-hydrogen bonding replacing halide-hydrogen bonding as α decreases. A Eu3+-doped Zr,Cl glass exhibits photoluminescence transitions 5D0 → 7Fn (n = 1, 2, and 4) of Eu3+, illustrating the asymmetric nature of the dopant sites in the glass.

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X-rays and Neutrons.

This Review addresses the use of X-ray and neutron scattering as well as X-ray absorption to describe how inorganic nanostructured materials assemble, evolve, and function in solution. We first provide an overview of techniques and instrumentation (both large user facilities and benchtop). We review recent studies of soluble inorganic nanostructure assembly, covering the disciplines of materials synthesis, processes in nature, nuclear materials, and the widely applicable fundamental processes of hydrophobic interactions and ion pairing. Reviewed studies cover size regimes and length scales ranging from sub-Ångström (coordination chemistry and ion pairing) to several nanometers (molecular clusters, i.e. polyoxometalates, polyoxocations, and metal-organic polyhedra), to the mesoscale (supramolecular assembly processes). Reviewed studies predominantly exploit 1) SAXS/WAXS/SANS (small- and wide-angle X-ray or neutron scattering), 2) PDF (pair-distribution function analysis of X-ray total scattering), and 3) XANES and EXAFS (X-ray absorption near-edge structure and extended X-ray absorption fine structure, respectively). While the scattering techniques provide structural information, X-ray absorption yields the oxidation state in addition to the local coordination. Our goal for this Review is to provide information and inspiration for the inorganic/materials science communities that may benefit from elucidating the role of solution speciation in natural and synthetic processes.

Metal-Oxo Cluster Formation Using Ammonium and Sulfate to Differentiate MIV (Th, U, Ce) Chemistries.

Isolating isostructural compounds of tetravalent metals MIV (Zr, Hf, Ce, Th, U, Pu, Np) improves our understanding of metal hydrolysis and coordination behavior across the periodic table. These metals form polynuclear clusters typified by the hexamer [MIV6O4(OH)4]12+. Exploiting the ammonium MIV-sulfate (CeIV, ThIV, and UIV) phase space targeting rapid crystallization, we isolate the common hexamer [MIV6(OH)4(O)4]12+ but with different numbers of capping sulfates and water molecules for CeIV, ThIV, and UIV. These phases allowed a direct comparison of bonding trends across the series. Upon cocrystallization with the hexamers, higher complex structures can be identified. Thorium features assemblies with monomer-linked hexamer chains. Uranium features assemblies with sulfate-bridged hexamers and the supramolecular assembly of 14 hexamers into the U84, [U6(OH)4(O)4)14(SO4)120(H2O)42]72-. Last, cerium showcases the isolation from monomers to the Ce62, [Ce62(OH)30(O)58(SO4)71(H2O)33.25]41-. Furthermore, small-angle X-ray scattering (room temperature) shows ammonium-induced cluster assembly for CeIV but minimal reactivity for UIV and ThIV. In this study, because the phases crystallized at elevated temperature demonstrates favorable cluster assembly, these solution phase results were surprising and suggest some other characteristics such as Ce's facile redox behavior, contributes to its solution-phase speciation.

Elucidating Actinide-Pertechnetate and Actinide-Perrhenate Bonding via a Family of Th-TcO4 and Th-ReO4 Frameworks and Solutions.

Technetium-99, a ß-emitter produced from 235U fission, poses a challenge for the nuclear industry due to co-extraction of pertechnetate (TcO4-) with the actinides (An) during nuclear fuel reprocessing. Previous studies suggested that direct coordination of pertechnetate with An plays an important role in the coextraction process. However, few studies have provided direct evidence for An-TcO4- bonding in the solid state, and even fewer in solution. The present study describes synthesis and structural elucidation of a family of thorium(IV)-pertechnetate/perrhenate (ReO4-, nonradioactive surrogate) compounds, which is obtained by dissolution of thorium oxyhydroxide in perrhenic/pertechnic acid followed by crystallization, with or without heating. For reaction ratios of 3:1, 4:1, and 6:1 MO4-/Th(IV) (M = Tc, Re), the crystallized compounds reflect the same ratio, suggesting facile and flexible coordination. Nine structures reveal 1-dimensional and 2-dimensional frameworks with varying topologies. While a multitude of compounds isolated from 4:1 (and 6:1) reaction solutions feature Th monomers linked by MO4-, the 3:1 reaction solution yielded the well-known dihydroxide-bridged thorium dimer, linked, and capped by MO4-. Density functional theory calculations on ReO4-/TcO4- isomorphs suggest similar bonding characteristics in the solid state, but experimental solution characterization noted differences. Specifically, small-angle X-ray scattering studies suggest the bonding of Th-TcO4- persists in solution, while Th-ReO4- bonding is less apparent.

Contrasting Trivalent Lanthanide and Actinide Complexation by Polyoxometalates via Solution-State NMR.

Deciphering the solution chemistry and speciation of actinides is inherently difficult due to radioactivity, rarity, and cost constraints, especially for transplutonium elements. In this context, the development of new chelating platforms for actinides and associated spectroscopic techniques is particularly important. In this study, we investigate a relatively overlooked class of chelators for actinide binding, namely, polyoxometalates (POMs). We provide the first NMR measurements on americium-POM and curium-POM complexes, using one-dimensional (1D) 31P NMR, variable-temperature NMR, and spin-lattice relaxation time (T1) experiments. The proposed POM-NMR approach allows for the study of trivalent f-elements even when only microgram amounts are available and in phosphate-containing solutions where f-elements are typically insoluble. The solution-state speciation of trivalent americium, curium, plus multiple lanthanide ions (La3+, Nd3+, Sm3+, Eu3+, Yb3+, and Lu3+), in the presence of the model POM ligand PW11O397- was elucidated and revealed the concurrent formation of two stable complexes, [MIII(PW11O39)(H2O)x]4- and [MIII(PW11O39)2]11-. Interconversion reaction constants, reaction enthalpies, and reaction entropies were derived from the NMR data. The NMR results also provide experimental evidence of the weakly paramagnetic nature of the Am3+ and Cm3+ ions in solution. Furthermore, the study reveals a previously unnoticed periodicity break along the f-element series with the reversal of T1 relaxation times of the 1:1 and 1:2 complexes and the preferential formation of the long T1 species for the early lanthanides versus the short T1 species for the late lanthanides, americium, and curium. Given the broad variety of POM ligands that exist, with many of them containing NMR-active nuclei, the combined POM-NMR approach reported here opens a new avenue to investigate difficult-to-study elements such as heavy actinides and other radionuclides.

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate.

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Oxo-Cluster-Based Zr/HfIV Separation: Shedding Light on a 70-Year-Old Process.

Zirconium and hafnium in the tetravalent oxidation state are considered the two most similar elements on the periodic table, based on their coexistence in nature and their identical solid-state chemistry. However, differentiating solution phase chemistry is crucial for their separation for nuclear applications that exploit the neutron capture of Hf and neutron transparency of Zr. Here we provide molecular level detail of the multiple factors that influence Zr/Hf separation in a long-exploited, empirically designed industrial solvent-extraction process that favors Hf extraction into an organic phase. In the aqueous solution, both Hf and Zr form an oxo-centered tetramer cluster with a core formula of [OM4(OH)6(NCS)12]4- (OM4-NCS, M = Hf, Zr). This was identified by single-crystal X-ray diffraction, as well as small-angle X-ray scattering (SAXS), of both the aqueous and organic phase. In addition to this phase, Zr also forms (1) a large oxo-cluster formulated [Zr48O30(OH)92(NCS)40(H2O)40] (Zr48) and (2) NCS adducts of OZr4-NCS. Zr48 was identified first by SAXS and then crystallized by exploiting favorable soft-metal bonding to the sulfur of NCS. While the large Zr48 likely cannot be extracted due to its larger size, the NCS adducts of OZr4-NCS are also less favorable to extraction due to the extra negative charge, which necessitates coextraction of an additional countercation (NH4+) per extra NCS ligand. Differentiating Zr and Hf coordination and hydrolysis chemistry adds to our growing understanding that these two elements, beyond simple solid-state chemistry, have notable differences in chemical reactivity.

Effective Storage of Electrons in Water by the Formation of Highly Reduced Polyoxometalate Clusters.

Aqueous solutions of polyoxometalates (POMs) have been shown to have potential as high-capacity energy storage materials due to their potential for multi-electron redox processes, yet the mechanism of reduction and practical limits are currently unknown. Herein, we explore the mechanism of multi-electron redox processes that allow the highly reduced POM clusters of the form {MO3}y to absorb y electrons in aqueous solution, focusing mechanistically on the Wells-Dawson structure X6[P2W18O62], which comprises 18 metal centers and can uptake up to 18 electrons reversibly (y = 18) per cluster in aqueous solution when the countercations are lithium. This unconventional redox activity is rationalized by density functional theory, molecular dynamics simulations, UV-vis, electron paramagnetic resonance spectroscopy, and small-angle X-ray scattering spectra. These data point to a new phenomenon showing that cluster protonation and aggregation allow the formation of highly electron-rich meta-stable systems in aqueous solution, which produce H2 when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [P5W30O110]15- and [P8W48O184]40- anions, which can be charged to 23 and 27 electrons per cluster, respectively.

Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters.

Aqueous solutions of lithium uranyl triperoxide, Li4[UO2(O2)3] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li24[(UO2)(O2)(OH)]24 (Li-U24). Raman spectroscopy and 18O labeling were used to identify the Raman-active vibrations of LiUT. With these assignments, the concentration of LiUT was tracked as a function of radiation dose. A discrepancy between monomer removal and cluster formation suggests that the reaction proceeds by the assembly of an intermediate. Non-negative matrix factorization was used to separate Raman spectra into components and resulted in the identification of a unique intermediate species. Much of the conversion appears to be driven by water radiolysis products, particularly the hydroxyl radical. This differs from the 18O-labeled copper-catalyzed formation of U24, which progresses at a steady rate with no observation of intermediates. Li-U24 in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na2(UO2)6O4(OH)6·7H2O, which contains uranyl oxyhydroxy sheets.

Nb2O5, LiNbO3, and (Na, K)NbO3 Thin Films from High-Concentration Aqueous Nb-Polyoxometalates.

Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of Nb2O5, LiNbO3, and (Na, K)NbO3 (KNN) from high-concentration solutions, up to 2.5 M Nb for Nb2O5 and ∼1 M Nb for LiNbO3 and KNN. Deposition of KNN from 1 M Nb concentration represents a potentially important advancment in lead-free piezoelectrics, an application that requires thick films. Solution characterization via small-angle X-ray scattering and Raman spectroscopy described the speciation for all precursor solutions as the [HxNb24O72](x-24) POM, as did total pair distribution function analyses of X-ray scattering of amorphous gels prior to conversion to oxides. The tendency of the Nb24-POM to form extended networks without crystallization leads to conformal and well-adhered films. The films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, ellipsometry, and X-ray photoelectron spectroscopy. As a strategy to convert aqueous deposition solutions from {Nb10}-POMs to {Nb24}-POMs, we devised a general procedure to produce doped Nb2O5 thin films including Ca, Ag, and Cu doping.

Solvent-Driven Transformation of Zn/Cd2+-Deoxycholate Assemblies.

Deoxycholic acid (DOC) is a unique, biologically derived surfactant with facial amphiphilicity that has been exploited, albeit minimally, in supramolecular assembly of materials. Here, we present the synthesis and structural characterization of three hybrid metal (Zn2+ and Cd2+)-DOC compounds. Analysis by single-crystal X-ray diffraction reveals the many interactions that are possible between these facial surfactants and the influence of solvent molecules that drive the assembly of materials. These structures are the first metal-DOC complexes besides those obtained from alkali and alkaline earth metals. We isolated polymeric chains of both Cd and Zn (Znpoly-DOC and Cdpoly-DOC) from water. Major interactions between DOC molecules in these phases are hydrophobic in nature. Cdpoly-DOC exhibits unique P1 symmetry, with complete interdigitation of the amphiphiles between neighboring polymeric chains. Zn4-DOC, obtained from methanol dissolution of Znpoly-DOC, features the OZn4 tetrahedron, widely known in basic zinc acetate and MOF-5 (metal organic framework). We document a solvent-driven, room-temperature transition between Znpoly-DOC and Zn4-DOC (in both directions) by scanning and transmission electron microscopies in addition to small-angle X-ray scattering, powder X-ray diffraction, and infrared spectroscopy. These studies show the methanol-driven transition of Znpoly-DOC to Zn4-DOC occurs via an intermediate with no long-range order of the Zn4 clusters, indicating the strongest interactions driving assembly are intramolecular. On the contrary, water-driven solid-to-solid transformation from Zn4-DOC to Znpoly-DOC exhibits crystal-to-crystal transformation. Znpoly-DOC is robust, easy to synthesize, and comprised of biologically benign components, so we demonstrate dye absorption as a proxy for water treatment applications. It favors absorption of positively charged dyes. These studies advance molecular level knowledge of the supramolecular assembly of facial surfactants that can be exploited in the design of organic-inorganic hybrid materials. This work also highlights the potential of solvent for tuning supramolecular assembly processes, leading to new hybrid materials featuring facial surfactants.

Radiation-Induced Solid-State Transformations of Uranyl Peroxides.

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca2[UO2(O2)3]·9H2O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca2+ is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca2+ cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice H2O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO2(O2)2(OH)24-, that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li4[(UO2)(O2)3]·10H2O results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and H2O in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li+ and Ca2+ countercations. Highly hydratable and mobile Li+ yields to uranyl hydrolysis reactions, while Ca2+ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.