Polyoxometalate Clusters Research Articles

Atomically Precise Ternary Cluster: Polyoxometalate Cluster Sandwiched by Gold Clusters Protected by N‐Heterocyclic Carbenes

AbstractCoinage metal (Au, Ag, Cu) cluster and polyoxometalate (POM) cluster represent two types of subnanometer “artificial atoms” with significant potential in catalysis, sensing, and nanomedicine. While composite clusters combining Ag/Cu clusters with POM have achieved considerable success, the assembly of gold clusters with POM is still lagging. Herein, we first designedly synthesized two cluster structural units: an Au3O cluster stabilized by diverse N‐heterocyclic carbene (NHC) ligands and an amine‐terminated POM linker. The subsequent reaction involved amine substitution in the POM linker for the central O atom in the Au3O cluster, resulting in the first ternary composite cluster—a POM cluster sandwiched by two Au clusters protected by NHCs. Single‐crystal X‐ray diffraction and other characteristic methods characterized their atomically precise structures. Furthermore, altering the NHC ligands decreased the number of gold atoms in the sandwich structures, accompanying the different protonated degrees of amine ligand in the terminal end of the POM linker. These composite clusters showed excellent performances in catalytic H2O2 conversion through the synergistic effect between gold clusters and POM clusters. This work opens a new avenue to functional composite metal clusters and would promote their enhanced catalysis applications through intercluster synergistic interactions within composite systems.

Construction of Spatially Separated Acid-Base Sites inside and outside Metal-Organic Framework Nanocrystals for Efficient Cascade Reactions.

The development of acid-base catalysts for one-pot cascade reactions remains challenging because of the inherent incompatibility of inorganic acid and base active sites. Here, we introduced an innovative approach that employs spatial separation to construct separated inorganic acid-base sites, achieving sequence control of acid-base cascade reactions. The as-prepared bifunctional catalyst applied metal-organic framework (MOF) nanocrystals as spatial separators, with the inside microcavities loaded with 1 nm inorganic polyoxometalate acid H3PMo12O40 clusters (NENU-5) and the outside crystal surface covered with basic CuCo layered double hydroxide (CuCo-LDH) nanosheets. By applying the resultant NENU-5@CuCo-LDH catalyst to cascade deacetalization-Knoevenagel condensation, over 99% of the benzaldehyde dimethyl acetal (BDMA) was transformed into the final benzylidene cyanoacetate (BCA) with a 99% yield at 80 °C and solvent-free conditions. Compared to the random NENU-5+CuCo-LDH, the NENU-5@CuCo-LDH with spatially separated acid-base sites indicated a higher reaction rate due to the designed reaction sequence and the shorter mass transfer pathway. Moreover, this hierarchical structure showed inherent shape selectivity and extraordinary stability. This study introduces spatial separation to address the incompatibility issue between inorganic acid and base active sites, offering novel perspectives for acid-base systems.

Homogeneous Integration of Polyoxometalates and Titania into Crumpled Layers.

The crumpling and buckling in nanosheets are anticipated to provide new characteristics that could not be observed in ideal flat layers. However, the rigid lattice structure of inorganic metal oxides limits their assembly into well-defined crumpled layers. Here, this study demonstrates that at the sub-nm scale, polyoxometalates (POMs) clusters having well-defined structures can intercede during the nucleation process of titania and co-assemble with nuclei to form uniform, large-sized crumpled binary 2D layers with a thickness of 2nm. The obtained crumpled layers are then used as a support material to immobilize Pd nanoclusters with an average size of 2nm. Pd-immobilized crumpled layers are employed as heterogeneous catalysts for the partial hydrogenation of acetylene. This structurally and compositionally unique heterogeneous catalyst manifests exceptional selectivity to cis-alkene with almost 100% yield as compared to commercially available titania which only exhibits 10% diphenylacetylene conversion and 42% selectivity in the given period of time.

Cluster-Cation Pairs Mediated Assembly of Subnanometer Polyoxometalates Superstructures.

Superstructures assembled by subnanometer polyoxometalate (POM) clusters are interesting for their attractive structures and excellent properties. However, the complex interactions between clusters and cations make it challenging to control the assembly of POM clusters at the subnanometer scale. Here, 20 cluster-assembled superstructures built by two types of MP2W17O61 (M = La-Lu) clusters are successfully synthesized. The precise structures and configurations of the subnanostructures, including nanowires, tetragonal nanosheets, and rectangular nanosheets, are characterized and presented. Molecular dynamics (MD) simulations reveal that the difference in interactions of POM clusters and cations leads to the formation of distinct superstructures. Two mechanisms of superstructure formation are proposed. Furthermore, the EuP2W17 nanosheet behaves with a high Faradaic efficiency of 90.2% and selectivity of 87.3% for glycolic acid in the electrocatalytic ethylene glycol oxidation reaction, which is much higher than that of isolated cluster components. This work connects the cluster topologies and cluster-cation pairs to the superstructures of cluster assemblies, providing general guidelines for the supramolecular self-assembly of POM clusters.

Atomically Precise Ternary Cluster: Polyoxometalate Cluster Sandwiched by Gold Clusters Protected by N-Heterocyclic Carbenes.

Coinage metal (Au, Ag, Cu) cluster and polyoxometalate (POM) cluster represent two types of subnanometer "artificial atoms" with significant potential in catalysis, sensing, and nanomedicine. While composite clusters combining Ag/Cu clusters with POM have achieved considerable success, the assembly of gold clusters with POM is still lagging. Herein, we first designedly synthesized two cluster structural units: an Au3O cluster stabilized by diverse N-heterocyclic carbene (NHC) ligands and an amine-terminated POM linker. The subsequent reaction involved amine substitution in the POM linker for the central O atom in the Au3O cluster, resulting in the first ternary composite cluster - a POM cluster sandwiched by two Au clusters protected by NHCs. Single-crystal X-ray diffraction and other characteristic methods characterized their atomically precise structures. Furthermore, altering the NHC ligands decreased the number of gold atoms in the sandwich structures, accompanying the different protonated degrees of amine ligand in the terminal end of the POM linker. These composite clusters showed excellent performances in catalytic H2O2 conversion through the synergistic effect between gold clusters and POM clusters. This work opens a new avenue to functional composite metal clusters and would promote their enhanced catalysis applications through intercluster synergistic interactions within composite systems.

Unprecedented Organogermanium Functionalized GeIV-SbIII-Templating Polyoxotungstate Nanocluster for Photothermal-Chemodynamic Cancer Therapy.

The function-oriented synthesis of polyoxometalate (POM) nanoclusters has become an increasingly important area of research. Herein, the well-known broad-spectrum anticancer drug Ge-132 which contains GeIV as potential heteroatoms and carboxyl coordination sites, is introduced to the POM system, leading to the first organogermanium functionalized GeIV-SbIII-templating POM nanocluster Na4[H2N(CH3)2]16 H18[Sm4(H2O)12W4O14Ge(CH2CH2COOH)]2[SbW9O33]4[Ge(CH2CH2COOH) SbW15O54]2·62H2O (1). An unprecedented organogermanium templating Dawson-like [Ge(CH2CH2COOH)SbW15O54]12- building block is discovered. To take advantage of the potential pharmaceutical activity of such an organogermanium-functionalized POM cluster, 1 is further composited with gold nanoparticles (NPs) to prepare 1-Au NPs, which doubles the blood circulation time of 1-based nanodrug. Efficient separation of photogenerated charges in 1-Au NPs largely boosts the photothermal conversion efficiency (PCE=55.0%), which is nearly 2.1 times that of either single 1 (PCE=26.7%) or Au NPs (PCE=26.2%), and simultaneously facilitate the generation of toxic activate reactive oxygen species in tumor microenvironment. Based on these findings, it is demonstrated that 1-Au NPs are a multifunctional and renal clearable nanomedicine with great potential in photoacoustic imaging guiding photothermal-chemodynamic therapy for breast cancer.

Upscaling Brønsted acid intercalation and exfoliation of graphite into graphene by polyoxometalate clusters for sodium-ion battery application

Non-oxidative intercalation of graphite avoids damage to graphene lattices and is a suitable method to produce high-quality graphene. However, the yield of exfoliated graphene is low in this process due to the poor delamination efficiency of guest species. In this study, a Brønsted acid intercalation protocol is developed involving polyoxometalate (POM) clusters (H6P2W18O62) as guests and intercalation of graphite is realized at the sub-nanometer scale. Theoretical simulation based on DFT elucidates the stepwise intercalation mechanism of Brønsted acid molecules and clusters. Unlike common molecules/ionic guests, intercalation of POM clusters induces large expansion and extensive donor–acceptor interactions among graphite interlayers. This significantly weakens the van der Waals forces and promotes exfoliation efficiency of graphene layers. The exfoliated graphene possesses outstanding features of large lateral size, thin thickness, and high purity, and shows excellent performance as the anode for high power sodium-ion batteries. This work proffers a new pathway toward non-oxidative intercalation of graphite for large-scale production of graphene.

Multistate nonpolar resistive switching in nickel embedded polyoxovanadate for high density data storage

The evolution of the electronic industry constantly relies on downscaling of electronic devices and integrating novel materials in active regions to accomplish ever-higher speeds and new features in device structures. Employing materials that display multistate switching for resistive-random-access-memory or simply resistive memory could be a simple and effective way to realize high density data storage. In this context, we report multistate “nonpolar” resistive switching in a nickel embedded polyoxovanadate cluster, (K2H5[NiV14O40]) – a molecule that belongs to the larger polyoxometalate family. We observed unique and distinctive nonpolar resistive switching behaviour for the first time in a multi-redox polyoxometalate cluster. The switching characteristics were repeatable for more than 200 cycles. Our two terminal Al/K2H5[NiV14O40])/ITO memory cells exhibited considerably high resistance window (105) and also long retention time (2000 s). This work holds promise for a novel strategy in order to achieve multilevel storage by exploiting different varieties of polyoxometalate molecules as active switching element that can possibly connect memory with neuromorphic computing.

Co-Assembly of Polyoxometalates and Porphyrins as Anode for High-Performance Lithium-Ion Batteries.

Polyoxometalates (POMs) have been considered one of the most promising anode candidates for lithium-ion batteries (LIBs) in virtue of their high theoretical capacity and reversible multielectron redox properties. However, the poor intrinsic electronic conductivity, low specific surface area, and high solubility in organic electrolytes hinder their widespread applications in LIBs. Herein, a novel hybrid nanomaterial is synthesized by co-assembling POMs and porphyrins (PMo12/CoTPyP) through a facile solvothermal method. The POM clusters are stabilized by porphyrin units through electrostatic interactions, which simultaneously realize the uniform dispersion of POMs and porphyrin units. Benefiting from the generated sub-1nm channels for fast ion transport and the synergistic effect between evenly distributed PMo12 clusters and high-conductive CoTPyP units, the LIB based on the optimized PMo12/CoTPyP anode exhibits significantly improved Li+ storage capability as well as superior rate and cycling performance. The results of density functional theory simulations further reveal that the co-assembly of PMo12 and CoTPyP can accelerate the mobility of Li+ and electrons, which in turn promotes the enhancement of LIBs performance. This work paves a strategy for synthesizing POMs-based anode materials with simultaneously high dispersibility, redox activity, and stability.

Polyoxometalate Functionalized Cyclic Trinuclear Copper Compounds for Bifunctional Electrochemical Detection and Photocatalytic Reduction of Cr(VI).

The design and intentional construction of crystalline materials containing two clusters with redox properties in one framework still remains challenging. Linking oxidative polyoxometalate (POM) clusters and a reductive cyclic trinuclear copper complex (Cu-CTC) to prepare stable catalysts is rarely reported. Herein, we successfully obtained two new polyoxometalate-based metal-organic compounds (POMOCs) [CuII3(PyCA)3(μ3-OH)(β-Mo8O26)0.5(H2O)2]·5H2O (1), [CuII3(PyCA)3(μ3-OH)]2(CuIIW12O40)[CuII(H2O)6] (2) (PyCA = 1H-pyrazole-4-carbaldehyde) by enabling precursors of Cu-CTC and POM cocrystallization in one pot via hydrothermal method. The [β-Mo8O26]4- cluster in compound 1 combined with Cu-CTC units to form a 1D structure, and the [CuW12O40]6- unit in compound 2 linked two Cu-CTC units to form a sandwich-like 0D structure. Also, Cu-CTC CuI3(PyCA)3·H2O (Cu3) was synthesized for performance comparison. A series of characterizations indicate that compound 1 is more conducive to electron transfer than compound 2. In addition, compounds 1 and 2 can act as bifunctional catalysts for the electrochemical detection and photocatalytic reduction of Cr(VI). Particularly, the photoreduction rates of Cr(VI) by compounds 1 and 2 are 96.7% and 96.3% for only 10 and 14 min under visible light, respectively, and it is better than that of Cu3 and most other reported photocatalysts. Furthermore, the active sites and mechanisms for electrochemical detection and photocatalytic reduction of Cr(VI) were discussed.

Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination.

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW12) intercalated reduced graphene oxide (rGO) membrane (rGO-PW12) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW12 result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW12 membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW12 membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW12 membranes with a thickness of 90nm not only exhibit a high-water permeance of up to 0.11790 L m-2 h-1 bar-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW12 and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.

Self‐Assembly of Polyoxometalate Clusters into Nanostructures with Different Dimensions

AbstractSelf‐assembly of polyoxometalate (POM) clusters offers a viable method for constructing nanostructures with tailored properties. Over the past few decades, POMs‐based nanostructures of varying dimensions, such as nanowires, nanobelts, nanosheets, and other superstructures, have been constructed. Additionally, these nanostructures show broad prospects in optical, electrochemical, catalytic, and mechanical applications. Particularly noteworthy are surface modifications, such as surfactant encapsulation, which yield amphiphilic POMs. These modifications render the POMs compatible with organic systems, significantly broadening their morphological and functional applicability. This concept provides an overview of recent advancements in nanostructures based on self‐assembly of surfactant encapsulated POM clusters, encompassing one‐dimensional (1D), two‐dimensional (2D), and three‐dimensional (3D) structures. Through surfactant encapsulation, POM clusters can be rendered compatible with various solvents, facilitating the formation of hybrid assemblies with diverse morphologies and unique properties. The structures, formation mechanisms, and properties of these sub‐nanometer assemblies are discussed from both experimental and theoretical perspectives. This concept aims to provide a new insight into the design and fabrication of nanostructures based on POM clusters.

Monodisperse Manganese-Vanadium-Oxo Clusters with Extraordinary Lithium Storage.

Although possessing well-defined nanostructures and excellent multi-electron redox properties, polyoxometalate clusters have poor intrinsic electrical conductivity and are prone to aggregation due to large surface energy, which makes them difficult to be fully utilized when applying as electrode materials for lithium-ion batteries. In this paper, monodisperse K7MnV13O38 (MnV13) clusters are achieved by rationally utilizing nano-sized high conductive carbon dots (CDs) as stabilizers. Benefiting from the fully exposed redox sites of MnV13 clusters (high utilization rate) and sufficient interfaces with carbon dots (extra interfacial energy storage), the optimized MnV13/10CDs anode delivers a high discharge capacity up to 1348 mAh g-1 at a current density of 0.1 A g-1 and exhibits superb rate/cycling capabilities. Density functional theory (DFT) calculations verify that ionic archway channels are formed between MnV13 and CDs, eliminating the bandgap and greatly improving the electron/ion conductivity of MnV13 and CDs. This paper paves a brand-new way for synthesis of monodisperse clusters and maximization of extra interfacial energy storage.

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH.

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 μmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Single‐cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH

AbstractSolar‐driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi‐electron transfer. Single‐cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells‐Dawson‐ and Keggin‐type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single‐cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface‐hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2‐1 exhibits the highest electron consumption rate of 1260 μmol g−1 for CO2‐to‐CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single‐dispersion of molecular cluster to enrich surface‐reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Hydrophobic Two-Dimensional Layered Superstructure of a Polyoxometalate Cluster as the Cathode Material for Aqueous Zinc-Ion Batteries.

Aqueous zinc-ion batteries hold promise for sustainable energy storage, yet challenges in finding high-performance cathode materials persist. Polyoxovanadates (POVs) are emerging as potential candidates due to their structural diversity and robust redox activity. Despite their potential, issues like dissolution in electrolytes, structural degradation, and byproduct accumulation persist. This work introduces a POV-based hydrophobic two-dimensional (2D) layered superstructure that addresses these challenges. The hydrophobic nature minimizes POV dissolution, enhancing structural stability and inhibiting phase transitions during cycling. The 2D arrangement ensures a larger surface area and improved electronic conductivity, resulting in faster kinetics and higher specific capacity. The superstructure demonstrates improved cycle life and an increased operating voltage, marking a significant advancement in POV-based cathode materials for aqueous zinc-ion batteries.

Tuning the Chirality Evolution in Achiral Subnanometer Systems by Judicious Control of Molecule Interactions.

Chirality evolution from molecule levels to the nanoscale in an achiral system is a fundamental issue that remains undiscovered. Here, we report the assembly of polyoxometalate (POM) clusters into chiral subnanostructures in achiral systems by programmable single-molecule interactions. Driven by the competing binding of Ca2+ and surface ligands, POM assemblies would twist into helical nanobelts, nanorings, and nanotubes with tunable helicity. Chiral molecules can be used to differentiate the formation energies of chiral isomers and immobilize the homochiral isomer, where strong circular dichroism (CD) signals are obtained in both solutions and films. Chiral helical nanobelts can be used as circularly polarized light (CPL) photodetectors due to their distinct chiroptic responsivity for right and left CPL. By the fine-tuning of interactions at single-molecule levels, the morphology and CD spectra of helical assemblies can be precisely controlled, providing an atomic precision model for investigation of the structure-chirality relationship and chirality manipulation at the nanoscale.

Bismuth Telluride Supported Sub-1 nm Polyoxometalate Cluster for High-Efficiency Thermoelectric Energy Conversion.

Size plays a crucial role in chemistry and material science. Subnanometer polyoxometalate (POM) clusters have gained attention in various fields, but their use in thermoelectrics is still limited. To address this issue, we propose the POM clusters as an effective second phase to enhance the thermoelectric properties of Bi0.4Sb1.6Te3. Thanks to their subnanometer size, POM clusters improve electrical transport behavior through the superposition of atomic orbitals and the interfacial scattering effect. Furthermore, their ultrasmall size strongly reduces thermal conductivity. Consequently, the introduction of a mere 0.1 mol % of POM into the Bi0.4Sb1.6Te3 matrix realizes a state-of-the-art zT value of 1.46 at 348 K, a 45% enhancement over Bi0.4Sb1.6Te3 (1.01), along with a maximum thermoelectric-conversion efficiency of the integrated module of 6.0%. The enhancement of carrier mobility and the suppression of thermal conduction achieved by introducing the subnanometer clusters hold promise for various applications, such as electronic devices and thermal management.

Oxygen-Independent Radiodynamic Therapy: Radiation-Boosted Chemodynamics for Reprogramming the Tumor Immune Environment and Enhancing Antitumor Immune Response.

Radiodynamic therapy (RDT) has emerged as a promising modality for cancer treatment, offering notable advantages such as deep tissue penetration and radiocatalytic generation of oxygen free radicals. However, the oxygen-dependent nature of RDT imposes limitations on its efficacy in hypoxic conditions, particularly in modulating and eliminating radioresistant immune suppression cells. A novel approach involving the creation of a "super" tetrahedron polyoxometalate (POM) cluster, Fe12-POM, has been developed for radiation boosted chemodynamic catalysis to enable oxygen-independent RDT in hypoxic conditions. This nanoscale cluster comprises four P2W15 units functioning as energy antennas, while the Fe3 core serves as an electron receptor and catalytic center. Under X-ray radiation, a metal-to-metal charge transfer phenomenon occurs between P2W15 and the Fe3 core, resulting in the valence transition of Fe3+ to Fe2+ and a remarkable 139-fold increase in hydroxyl radical generation compared to Fe12-POM alone. The rapid generation of hydroxyl radicals, in combination with PD-1 therapy, induces a reprogramming of the immune environment within tumors. This reprogramming is characterized by upregulation of CD80/86, downregulation of CD163 and FAP, as well as the release of interferon-γ and tumor necrosis factor-α. Consequently, the occurrence of abscopal effects is facilitated, leading to significant regression of both local and distant tumors in mice. The development of oxygen-independent RDT represents a promising approach to address cancer recurrence and improve treatment outcomes.

Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters

Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters