Polyoxometalate Clusters Research Articles

Tuning the inner- and outer-sphere electron transfer behavior of aqueous {CoW12} polyoxometalate clusters for redox flow batteries exceeding 1.5V

Redox active species as the key component of redox flow batteries (RFBs) directly determine battery performance. Herein, we report a unique redox active specie, {CoW12}, exhibits both outer-sphere electron transfer of the {CoO4} core and inner-sphere electron transfer of the {W12O36} shell in one cluster, which is capable of constructing a symmetric {CoW12}-based RFB with pH-controlled operating voltage output. The DFT calculation discloses the proton-coupled electron transfer process of the {W12O36} shell in thermodynamically and kinetically, explaining the negative shift of the redox potential of {W12O36} shell as the pH increasing from 1 to 4. Interestingly, the redox potential of outer-sphere controlled {CoO4} core is unchanged, which leads to an extension of voltage window from 1.31 to 1.56 V. Consequently, the cycling measurement of the symmetrical battery proves that low capacity decay rate of 0.009 % and high Coulombic efficiency over 99 % can be retained in 400 cycles at pH 4.

Mechanically tunable organogels from highly charged polyoxometalate clusters loaded with fluorescent dyes

Inorganic nanowires-based organogel, a class of emerging organogel with convenient preparation, recyclability, and excellent mechanical properties, is in its infancy. Solidifying and functionalizing nanowires-based organogels by designing the gelator structure remains challenging. Here, we fabricate Ca2-P2W16 and Ca2-P2W15M nanowires utilizing highly charged [Ca2P2W16O60]10− and [Ca2P2W15MO60]14−/13− cluster units, respectively, which are then employed for preparing organogels. The mechanical performance and stability of prepared organogels are improved due to the enhanced interactions between nanowires and locked organic molecules. Compressive stress and tensile stress of Ca2-P2W16 nanowires-based organogel reach 34.5 and 29.0 kPa, respectively. The critical gel concentration of Ca2-P2W16 nanowires is as low as 0.28%. Single-molecule force spectroscopy confirms that the connections between cluster units and linkers can regulate the flexibility of nanowires. Furthermore, the incorporation of fluorophores into the organogels adds fluorescence properties. This work reveals the relationships between the microstructures of inorganic gelators and the properties of organogels, guiding the synthesis of high-performance and functional organogels.

Realizing high-performance glucose sensing in sweat: Synergistic use of nickel oxide nanosheets as photoelectrodes and the masking effect of Mo-POM for photoelectrochemical detection

The detection of blood glucose plays a crucial role in clinical practice, and the innovative use of sweat as a substitute for blood has brought significant practical value in the detection of blood glucose. However, lactic acid (LA) has been found to be the main interfering substance on the detection of glucose in sweat. In this study, a photoelectrochemical sensing system was constructed by using NiO nanosheets as photosensitive materials and molybdenum-based polyoxometalate clusters (Mo-POM) as masking agents. Mo-POM, an ideal electron-capturing agent, effectively promoted the spatial separation of photo-generated charge carriers in NiO nanosheets, thereby enhancing the oxidation rate of glucose on the valence band of NiO and significantly improving the detection sensitivity. Mo-POM also exhibited the characteristics of an electron sponge, facilitating fast and reversible multi-electron transfer reactions and effectively masking certain redox substances, especially lactic acid, thus greatly enhancing the selectivity of the glucose sensor. The photoelectrochemical (PEC) sensor constructed using Mo-POM has excellent sensing capability with high sensitivity to detect glucose at a minimum concentration of 1 nM. The detection limit of the sensor is 0.33 nM (S/N = 3). This sensor can be reliably applied to the detection of glucose in sweat and the accuracy of the results was verified by gas chromatography-mass spectrometry.

Catalytic Sequences and Kinetic Requirements for C–O Bond Scission and Formation in Methanol and Trioxane on Solid Polyoxometalate Clusters

Catalytic Sequences and Kinetic Requirements for C–O Bond Scission and Formation in Methanol and Trioxane on Solid Polyoxometalate Clusters

Improved supercapacitors and water splitting performances of Anderson-type manganese(III)-polyoxomolybdate through assembly with Zn-MOF in a host–guest structure

Enhancing performance through the combination of polyoxometalates (POMs) clusters with metal–organic frameworks (MOFs) that contain various transition metals is a challenging task. In this study, we synthesized a polyoxometalate-based metal–organic framework (POMOF) named HRBNU-5 using a solvothermal method. HRBNU-5 is composed of Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]@Zn3(C9H3O6)2·6C3H7NO, which includes two components: Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]·3C3H7NO ({Zn[MnMo6]}) and Zn3(C9H3O6)2·3C3H7NO (Zn-BTC). Structural characterization confirmed the host–guest structure, with Zn-BTC encapsulating {Zn[MnMo6]}. In a three-electrode system, HRBNU-5 exhibited a specific capacitance of 851.3 F g−1 at a current density of 1 A/g and retained high stability (97.2 %) after 5000 cycles. Additionally, HRBNU-5 performed well in aqueous-symmetric/asymmetric supercapacitors (SSC/ASC) in terms of energy density and power density in a double-electrode system. Moreover, it demonstrated excellent catalytic performance in a 1.0 M KOH solution, with low overpotentials and Tafel slopes for hydrogen and oxygen evolution reactions: 177.1 mV (η10 HER), 126.9 mV dec–1 and 370.3 mV (η50 OER), 36.3 mV dec–1, respectively, surpassing its precursors and most reported studies. HRBNU-5′s positive performance is attributed to its host–guest structure, high electron-transfer conductivity, and porous structure that enhances efficient mass transport. This work inspires the design of Anderson-type POMOF electrode materials with multiple active sites and a well-defined structure.

Sorting of Cluster-Confined Metallic Single-Walled Carbon Nanotubes for Fabricating Atomically Vacant Uranium Oxide.

Single-walled carbon nanotube (SWCNT) heterostructures have shown great potential in catalysis, magnetism, and nanofluidics, in which host SWCNTs with certain conductivity (metallic or semiconducting) are highly required. Herein, inspired by the large molecular weight and redox properties of polyoxometalate (POM) clusters, we reported the selective separation of POM encapsulated metallic SWCNTs (POM@m-SWCNTs) with a uniform diameter through density gradient ultracentrifugation (DGU). The confined POMs increased the SWCNT density and amplified the nanotubes' density difference, thus greatly lowering the centrifugal force (70,000g) of DGU. With this strategy, a series of POM@m-SWCNTs of ∼1.2 nm with high purity were sorted. The mechanism supported by theoretical and experimental evidence showed that the separation of m-SWCNTs depended on not only nanotube/cluster size but also the conductivity of SWCNTs. The smallest 1.2 nm m-SWCNT that can exactly accommodate a 0.9 nm-{Mo6} cluster exhibited the maximum electron transfer to inner clusters; thus, intertube π-π stacking of such m-SWCNTs was greatly loosened, leading to the preferential dispersion into individual ones and partitioning in the uppermost layer after DGU. As a proof-of-concept application, this sorting strategy was extended to separate heavy-element 238U-encapsulated m-SWCNTs. The phase-pure, tiny (1-2.5 nm) U4O9 crystals with atomic vacancy clusters were fabricated on m-SWCNTs through growth kinetic control. This work may provide a general way to construct desired actinide materials on specific SWCNTs.

The Atomic Arrangement of the Sherwoodite Structural Unit: Redefinition of the Sherwoodite Vanadoaluminate Heteropoly Complex as [AlV4+V5+12O39]11−

Abstract Sherwoodite, putatively Ca4.5(AlV5+12V4+2O40)·28H2O, was among the early naturally occurring polyoxometalates to be reported from the Uravan Mineral Belt; the ideal formula of sherwoodite is here redefined as Ca5.5[AlV4+V5+12O39]·28H2O. In an earlier preliminary structure report (R = 22%), researchers defined the composition of vanadoaluminate heteropoly complex in sherwoodite as (AlV4+,5+14O40)n−, the first natural occurrence of that polyoxometalate cluster. The cluster forms a framework around intermolecular channels that contain disordered arrays of H2O and charge-balancing Ca ions. A detailed structure study described herein [a = 27.9085(7), c = 13.8090(5), space group I41/amd, R1 < 6%] reveals new details of the vanadoaluminate heteropoly complex, indicating that it crystallizes as a structural variant of the putative (AlV4+,5+14O40)n− vanadoaluminate complex, [AlV4+V5+12O39]11−. That variant is derived through the random removal of one of the two apical vanadate octahedra, elucidating the cause of difficulties in the refinement of the preliminary structure report. The [AlV4+V5+12O39]11− complex is capped on one end by a Ca atom that bonds to four octahedral apices in a half-occupied site, and on the opposite end by the V1 and O1 atoms in half-occupied sites, similar to the compound potassium 13-vanadanomangate(IV). The capping octahedron is the locus of the V4+ ions in the mixed-valence complex. New electron microprobe analyses yielded an empirical formula, on the basis of (V4+V5+12), of (Ca4.26Mg1.09Na0.35K0.04Sr0.03)[Al0.87(V4+V5+12)O39]·28.15 H2O.

(Invited) Computational Modelling of Super-Reduced Polyoxometalates for Energy Storage

It is well know that polyoxometalates can be reduced at relatively potentials and can act as reservoir of electrons. Very recently, it has been reported that polyoxometalates clusters like Li6[P2W18O62] can achieve very high proton–electron storage capacities in aqueous solution. DFT calculations in combination with classical MD and CPMD simulations were able to substantiate that the physical properties of classical polyoxotungstates, like those of the prototypical Dawson anion [P2W18O62]6-, strongly depend on the presence of collective forces associated to POM concentration, pH and countercation nature. At low POM concentration and neutral pH only four electrons are reversibly incorporated to the POM. However, in acidic media, the POM framework begins to protonate after the third electron reduction. The salt concentration is also very relevant, since it favours the POM aggregation, which induces an additional stabilization of molecular orbitals. These and another features of reduced polyoxometales will be discussed in the present communication.

Ce-mediated molecular tailoring on gigantic polyoxometalate {Mo132} into half-closed {Ce11Mo96} for high proton conduction

Precise synthesis of polyoxometalates (POMs) is important for the fundamental understanding of the relationship between the structure and function of each building motif. However, it is a great challenge to realize the atomic-level tailoring of specific sites in POMs without altering the major framework. Herein, we report the case of Ce-mediated molecular tailoring on gigantic {Mo132}, which has a closed structural motif involving a never seen {Mo110} decamer. Such capped wheel {Mo132} undergoes a quasi-isomerism with known {Mo132} ball displaying different optical behaviors. Experiencing an ‘Inner-On-Outer’ binding process with the substituent of {Mo2} reactive sites in {Mo132}, the site-specific Ce ions drive the dissociation of {Mo2*} clipping sites and finally give rise to a predictable half-closed product {Ce11Mo96}. By virtue of the tailor-made open cavity, the {Ce11Mo96} achieves high proton conduction, nearly two orders of magnitude than that of {Mo132}. This work offers a significant step toward the controllable assembly of POM clusters through a Ce-mediated molecular tailoring process for desirable properties.

Hybrid Sub-1 nm Nanosheets of Co-assembled MnZnCuOx and Polyoxometalate Clusters as Anodes for Li-ion Batteries.

Transition metal oxide (TMO) anode materials in lithium-ion batteries (LIBs) usually suffer from serious volume expansion leading to the pulverization of structures, further giving rise to lower specific capacity and worse cycling stability. Herein, by introducing polyoxometalate (POM) clusters into TMOs and precisely controlling the amount of POMs, the MnZnCuOx -phosphomolybdic acid hybrid sub-1 nm nanosheets (MZC-PMA HSNSs) anode is successfully fabricated, where the special electron rich structure of POMs is conducive to accelerating the migration of lithium ions on the anode to obtain higher specific capacity, and the non-covalent interactions between POMs and TMOs make the HSNSs possess excellent structural and chemical stability, thus exhibiting outstanding electrochemical performance in LIBs, achieving a high reversible capacity (1157 mAh g-1 at 100 mA g-1 ) and an admirable long-term cycling stability at low and high current densities.

Hybrid Sub‐1 nm Nanosheets of Co‐assembled MnZnCuOx and Polyoxometalate Clusters as Anodes for Li‐ion Batteries

AbstractTransition metal oxide (TMO) anode materials in lithium‐ion batteries (LIBs) usually suffer from serious volume expansion leading to the pulverization of structures, further giving rise to lower specific capacity and worse cycling stability. Herein, by introducing polyoxometalate (POM) clusters into TMOs and precisely controlling the amount of POMs, the MnZnCuOx‐phosphomolybdic acid hybrid sub‐1 nm nanosheets (MZC‐PMA HSNSs) anode is successfully fabricated, where the special electron rich structure of POMs is conducive to accelerating the migration of lithium ions on the anode to obtain higher specific capacity, and the non‐covalent interactions between POMs and TMOs make the HSNSs possess excellent structural and chemical stability, thus exhibiting outstanding electrochemical performance in LIBs, achieving a high reversible capacity (1157 mAh g−1 at 100 mA g−1) and an admirable long‐term cycling stability at low and high current densities.

Frontispiece: Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Frontispiece: Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Frontispiz: Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Frontispiz: Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Biofilm Microenvironment-Responsive Self-Assembly Nanoreactors for All-Stage Biofilm Associated Infection through Bacterial Cuproptosis-like Death and Macrophage Re-Rousing.

Bacterial biofilm-associated infections (BAIs) are the leading cause of prosthetic implant failure. The dense biofilm structure prevents antibiotic penetration, while the highly acidic and H2 O2 -rich biofilm microenvironment (BME) dampens the immunological response of antimicrobial macrophages. Conventional treatments that fail to consistently suppress escaping planktonic bacteria from biofilm result in refractory recolonization, allowing BAIs to persist. Herein, a BME-responsive copper-doped polyoxometalate clusters (Cu-POM) combination with mild photothermal therapy (PTT) and macrophage immune re-rousing for BAI eradication at all stages is proposed. The self-assembly of Cu-POM in BME converts endogenous H2 O2 to toxic ·OH through chemodynamic therapy (CDT) and generates a mild PTT effect to induce bacterial metabolic exuberance, resulting in loosening the membrane structure of the bacteria, enhancing copper transporter activity and increasing intracellular Cu-POM flux. Metabolomics reveals that intracellular Cu-POM overload restricts the TCA cycle and peroxide accumulation, promoting bacterial cuproptosis-like death. CDT re-rousing macrophages scavenge planktonic bacteria escaping biofilm disintegration through enhanced chemotaxis and phagocytosis. Overall, BME-responsive Cu-POM promotes bacterial cuproptosis-like death via metabolic interference, while also re-rousing macrophage immune response for further planktonic bacteria elimination, resulting in all-stage BAI clearance and providing a new reference for future clinical application.

A monolayer crystalline covalent network of polyoxometalate clusters

Monolayer two-dimensional (2D) materials are of great interest because of their unique electronic structures, noticeable in-plane confinement effect, and exceptional catalytic properties. Here, we prepared 2D covalent networks of polyoxometalate clusters (CN-POM) featuring monolayer crystalline molecular sheets, formed by the covalent connection between tetragonally arranged POM clusters. The CN-POM shows a superior catalytic efficiency in the oxidation of benzyl alcohol, and the conversion rate is five times higher than that of the POM cluster units. Theoretical calculations show that in-plane electron delocalization of CN-POM contributes to easier electron transfer and increases catalytic efficiency. Moreover, the conductivity of the covalently interconnected molecular sheets was 46 times greater than that of individual POM clusters. The preparation of monolayer covalent network of POM clusters provides a strategy to synthesize advanced cluster-based 2D materials and a precise molecular model to investigate the electronic structure of crystalline covalent networks.

Designed hybrid organic−inorganic nanocomposite film based on synergistic effect of conducting polymer and keggin type polyoxometalate clusters

The exchange of electrical charges between a chemical reaction center and an external electrical circuit is critical for many advanced technological applications. From this perspective, "wiring” of polyoxometalates (POM) which is highly redox-active molecular metal oxide anions to conductive polymers (CPs) will be of great importance. Herein, POM-CP-based organic-inorganic hybrid composite film has been prepared electrochemically from mixtures of Keggin-type POM clusters (K7-xNaxPW11O39∙14H2O) and a triazine cored carbazole derivative. Incorporating POM clusters into cross-linked CPs has led to hybrid material with not only value-adding properties but also synergistic effects. When optical and electrical properties of the composite films prepared from different feed ratios have been examined, it has been observed that the incorporation of POM clusters into the conductive polymer structure has led to a decrease in the onset potential, response time, and bandgap as well as an increase in the optical contrast, stability, and charge capacity of the CPs.

A Calix[4]resorcinarene-Copper(II) Based Supramolecular Nanocapsule with Encapsulated Polyoxometalates for Enhanced Photocatalytic Activity

The research on improving the photocatalytic activity of polyoxometalates (POMs) has attracted considerable attention. In this work, a functionalized calix[4]resorcinarene ligand with four terpyridine groups (TPYR4A) was rationally designed, and then a POM-calix[4]resorcinarene based inorganic–organic complex [Cu4Cl4(TPYR4A)]2[PWVI10WV2O40][PWVI12O40]·6DMA·8H2O (DMA= N,N′-dimethylacetamide) (1) was achieved by a self-assembly process. Strikingly, POM clusters were successfully encapsulated in noncovalent dimeric calix[4]resorcinarene-copper(II) based nanocapsules with a size of 3 nm. Markedly, 1 features a visible light absorption response with an optical band gap of 2.3 eV. More importantly, 1 exhibits obviously enhanced photocatalytic activity for oxidation of benzaldehyde upon simulated solar light excitation. Particularly, the photocatalytic performance of 1 is better than that of pure H3PW12O40 or a mixture of TPYR4A and H3PW12O40. This work offers a feasible strategy for designing POM-based hybrid materials with enhanced photocatalytic activity.

Hydroxylated Polyoxometalate with Cu(II)- and Cu(I)-Aqua Complexes: A Bifunctional Catalyst for Electrocatalytic Water Splitting at Neutral pH.

A sole inorganic framework material [Li(H2O)4][{CuI(H2O)1.5} {CuII(H2O)3}2{WVI12O36(OH)6}]·N2·H2S·3H2O (1) consisting of a hydroxylated polyoxometalate (POM) anion, {WVI12O36(OH)6}6-, a mixed-valent Cu(II)- and Cu(I)-aqua cationic complex species, [{CuI(H2O)1.5}{CuII(H2O)3}2]5+, a Li(I)-aqua complex cation, and three solvent molecules, has been synthesized and structurally characterized. During its synthesis, the POM cluster anion gets functionalized with six hydroxyl groups, i.e., six WVI-OH groups per cluster unit. Moreover, structural and spectral analyses have shown the presence of H2S and N2 molecules in the concerned crystal lattice, formed from "sulfate-reducing ammonium oxidation (SRAO)". Compound 1 functions as a bifunctional electrocatalyst exhibiting oxygen evolution reaction (OER) by water oxidation and hydrogen evolution reaction (HER) by water reduction at the neutral pH. We could identify that the hydroxylated POM anion and copper-aqua complex cations are the functional sites for HER and OER, respectively. The overpotential, required to achieve a current density of 1 mA/cm2 in the case of HER (water reduction), is found to be 443 mV with a Faradaic efficiency of 84% and a turnover frequency of 4.66 s-1. In the case of OER (water oxidation), the overpotential needed to achieve a current density of 1 mA/cm2 is obtained to be 418 mV with a Faradaic efficiency of 80% and turnover frequency of 2.81 s-1. Diverse electrochemical controlled experiments have been performed to conclude that the title POM-based material functions as a true bifunctional catalyst for electrocatalytic HER as well as OER at the neutral pH without catalyst reconstruction.

Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis.

Cluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B-α-PW9 O34 )Co3 (OH)(H2 O)2 (O3 PC(O)-(C3 H6 NH3 )PO3 )}2 Co]14- (Co7 ) and formation of a series of POM-based solid catalysts with the counter-cations Ag+ , Cs+ , Sr2+ , Ba2+ , Pb2+ , Y3+ , and Ce3+ . They exhibit improved catalytic activities for visible-light-driven water oxidation following the trend CsCo7 >SrCo7 >AgCo7 >CeIII Co7 >BaCo7 >YCo7 >PbCo7 . While CsCo7 exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo7 is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real-time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier-transform infrared spectroscopy, electron microscopy, X-ray diffraction patterns, Raman spectroscopy, X-ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.

Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

AbstractCluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B‐α‐PW9O34)Co3(OH)(H2O)2(O3PC(O)‐(C3H6NH3)PO3)}2Co]14− (Co7) and formation of a series of POM‐based solid catalysts with the counter‐cations Ag+, Cs+, Sr2+, Ba2+, Pb2+, Y3+, and Ce3+. They exhibit improved catalytic activities for visible‐light‐driven water oxidation following the trend CsCo7>SrCo7>AgCo7>CeIIICo7>BaCo7>YCo7>PbCo7. While CsCo7 exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo7 is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real‐time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier‐transform infrared spectroscopy, electron microscopy, X‐ray diffraction patterns, Raman spectroscopy, X‐ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.