Polyoxometalate Molecules Research Articles

Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States.

The 2-bit Lindqvist-type polyoxometalate (POM) [V6O13((OCH2)3CCH2N3)2]2- with a diamagnetic {V6O19} core and azide termini shows six fully oxidized VV centers in solution as well as the solid state, according to 51V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S0 state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S0 to S9 state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of VV into VIV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO2//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T1 state of the POM is reached.

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.

Confined grotthuss proton-conduction along polyoxometalate chains inside carbon nanotubes for high-rate charge storage

Redox-active materials with excellent rate capability and cycling performance are in highly demand for electrochemical energy storage and conversion applications. Here, we unveil the confined proton-conduction behavior of one-dimensional polyoxometalate chains inside single-walled carbon nanotubes (SWNTs). The polyoxometalate molecules including phosphomolybdic acid {HPMo} and phosphotungstic acid {HPW} are encapsulated within SWNTs via host–guest recognition, driven by the electron transfer from nanotubes to polyoxometalates. Impressively, the redox hybrids of polyoxometalate@SWNTs deliver abnormal rate performance in acidic solution with capacity retentions over 73 % at high sweep rate of 1000 mV s−1, relative to the capacity at 10 mV s−1. This value is comparable to that of the pure SWNTs, typically known as double-layer capacitive materials. Electrochemical analyses reveal the Grotthuss proton-conduction mechanism of {HPMo} in SWNTs with a low activation energy of 0.11 eV. Therefore, the {HPMo}@SWNT hybrid demonstrates quasi diffusion-free characteristic with dominant capacitive contribution over 60 % at different sweep rates. Molecular dynamic simulations and density functional theory calculations evidence the strong burst-like transport of protons in the {HPMo}@SWNT hybrid. This process is dynamically favored in the continuous and long-range hydrogen-bond network along polyoxometalate chains with bound/free water molecules inside SWNTs. The superiority of nanoconfined Grotthuss mechanism may engender an exciting avenue for developing high-performance energy storage materials.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion.

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

The superior HDS catalyst derived from a new Waugh Co-Mo polyoxometalate via cationic substitution

In this paper, the Waugh Co-Mo polyoxometalate (POM) (NH4)6[CoMo9O32] was prepared and cationic substituted to increase the Co/Mo ratio to 0.44 with Mo and Co atoms in a new Waugh Co-Mo POM molecule. The method was verified by X-ray diffraction, X-ray fluorescence, and infrared spectroscopy. Then, two series of Co-Mo hydrodesulfurization (HDS) catalysts were prepared using the Waugh Co-Mo POMs and traditional precursors, respectively. The HDS evaluation results showed the activities of catalysts prepared from the new Waugh Co-Mo POM is higher than the reference catalysts. For the catalyst prepared from the new Waugh Co-Mo POM, the cationic substitution provides a higher Co/Mo ratio and atomic-level proximity between Mo and Co atoms, leading to shorter and less stacked MoS2 nano-particles and more exposure of unsaturated edges and corner sites. In addition, a weak interaction between the new Waugh Co-Mo POM and Al2O3 leads to more CoMoS active phases. Catalysts prepared from the new Waugh Co-Mo POM showed excellent HDS performance, demonstrating the new Waugh Co-Mo POM is a superior precursor to construct the efficient HDS catalyst.

Construction of high content CoMoS active sites through the combination of Co5Mo10 molecular precursors and inert carbon matrix support for deep hydrodesulfurization

The rational design of high sulfurization and abundant CoMoS active phases catalysts is the key to drive the industrialization of hydrodesulfurization (HDS). Herein, we prepared a nano-catalyst with well-dispersed and numerous CoMoS active sites through confining Co3(II)[Co2(III)Mo10O38H4] (Co5Mo10) polyoxometalates (POMs) molecules with high Co/Mo ratio in hollow mesoporous carbon spheres (HMCS). On the one hand, POMs acted as pre-assembled molecular platform can build and regulate the structure of active phases by precisely adjusting the Co/Mo atomic ratio at molecular level. Meanwhile, the intimate interaction of Co-Mo on the POMs precursor also ensures that the Co species mainly exists as Co-promoted MoS2 active phase after the sulfidation process, thus the highly efficient synergism of the bimetals can be fully utilized in the reaction. On the other hand, the mesoporous carbon sphere with homogeneous pore size and high specific surface area can effectively confine the growth of the catalyst to disperse the active metals. The weakening metal–metal interaction property of such inert support enables the S-Co5Mo10/HMCS-10 catalyst to realize higher sulfurization, shorter slab length and higher stacking number. In addition, the abundant mesopores and high pore capacity of support greatly facilitate the mass transfer process and provide sufficient space for the HDS reaction as the nanoreactor. The unique combination of POMs and HMCS enabled the S-Co5Mo10/H10 catalyst to achieve up to 99.2% conversion of dibenzothiophene (DBT) after 4 h of HDS reaction, while obtaining a TOF value of 1.3 × 10−3 s−1 (calculated when keeping the low conversion at 42% after 0.5 h of reaction). The structural orientation of the POMs precursors to the active phases of HDS catalysts was investigated using various characterization methods, and the structure–activity relationship between the catalytic performance and the sulfide active phases was also revealed in this work.

Application of polyoxometalates in third-generation solar cells

Researchers pursuing the development of third-generation solar cells, which typically include quantum dot-sensitized solar cells (QDSSCs), dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), and organic solar cells (OSCs), continue to prioritize low cost, simple preparation, high efficiency, and stability. Polyoxometalates (POMs) are a class of inorganic anionic metallic oxygen cluster compounds with abundant charge, skeleton structure, and excellent physical and chemical properties, such as strong electron acceptability, adjustable energy band structure, and reversible multi-electron redox properties. They are also inexpensive and environmentally friendly. Owing to these exceptional characteristics, POMs are used as building blocks for synthesizing other nanomaterials. Notably, leveraging the extraordinary characteristics of POMs to reduce costs and improve the efficiency and stability of solar cells is an effective strategy for addressing the current energy crisis. In this review, we summarize the research progress of various POM molecules and their derived POM-based nanomaterials in enhancing the performance of third-generation solar cells. Promising development prospects of POMs in the field of photovoltaic devices are also highlighted.

Heteroatom-Modulated Assembly of Hexalanthanoid-Containing Polyoxometalate-Based Coordination Networks.

Two series of lanthanoid (Ln)-containing polyoxometalates (POMs) {[Ln6(ampH)4(H2O)24-n(ampH2)n(PW11O39)2]·21H2O (Ln = Tb, n = 0 (1), Ln = Er, n = 1 (2)) and K2[Ln6(ampH)4(H2O)22(SiW11O39)2]·23H2O (Ln = Tb (3), Er (4)) (ampH2 = (aminomethyl) phosphonic acid)} have been synthesized with tri-lacunary Keggin-type POMs containing different types of heteroatoms. Compounds 1 and 2 display neutral organic-inorganic hybrid POM molecules containing {Ln6(ampH)4} ({Ln6}) cores sandwiched by two {PW11O39} units. By changing the heteroatoms from PV to SiIV, the extended 2D networks of 3 and 4 were successfully isolated where the adjacent {Ln6} clusters were connected by {SiW11O39} moieties. Luminescence performances and magnetic properties of 1-4 have been systematically surveyed. The solid-state fluorescence spectra of 1-4 display characteristic emissions of Ln components resulting from the 4f-4f transitions, and energy transfer from the POM segments to Ln3+ centers in 1 and 3 has been observed based on the lifetime decay behaviors. Furthermore, all compounds can be utilized as electrocatalysts toward reduction of nitrite with high stability.

Multiscale assembly of polyoxometalates: From clusters to materials

Polyoxometalates (POMs) have drawn broad and intense interest due to their unique structure-dependent properties. The assembly of POM molecules into nanoarchitectures and superstructures can bridge the gap between micro-molecule and bulk POM materials and integrate the merits of molecule and materials that are widely used in catalysis, energy conversion, electronics, and bioapplications. The synthetic methodologies and applications of multiscale POM assemblies have been widely studied. In this review, we survey the POM-based multiscale assembly focusing on two aspects: the strategies for multiscale assembly of POMs and the emerging applications of POM assemblies. For the multiscale assembly, we focus on the methods of POMs assembly including templated synthesis and self- and co-assembly of POMs into nanoarchitectures and superstructures. For the applications, we review the applications of POM assemblies in energy, catalysis, and functional materials. Finally, we present the prospects for the future synthetic design and applications of POM assemblies.

Host-Guest Chemistry in Boron Nitride Nanotubes: Interactions with Polyoxometalates and Mechanism of Encapsulation.

Boron nitride nanotubes (BNNTs) are an emerging class of molecular container offering new functionalities and possibilities for studying molecules at the nanoscale. Herein, BNNTs are demonstrated as highly effective nanocontainers for polyoxometalate (POM) molecules. The encapsulation of POMs within BNNTs occurs spontaneously at room temperature from an aqueous solution, leading to the self-assembly of a POM@BNNT host-guest system. Analysis of the interactions between the host-nanotube and guest-molecule indicate that Lewis acid-base interactions between W═O groups of the POM (base) and B-atoms of the BNNT lattice (acid) likely play a major role in driving POM encapsulation, with photoactivated electron transfer from BNNTs to POMs in solution also contributing to the process. The transparent nature of the BNNT nanocontainer allows extensive investigation of the guest-molecules by photoluminescence, Raman, UV-vis absorption, and EPR spectroscopies. These studies revealed considerable energy and electron transfer processes between BNNTs and POMs, likely mediated via defect energy states of the BNNTs and resulting in the quenching of BNNT photoluminescence at room temperature, the emergence of new photoluminescence emissions at cryogenic temperatures (<100 K), a photochromic response, and paramagnetic signals from guest-POMs. These phenomena offer a fresh perspective on host-guest interactions at the nanoscale and open pathways for harvesting the functional properties of these hybrid systems.

Polyoxometalate as electron acceptor in dye/TiO2 films to accelerate room-temperature NO2 gas sensing

It is an effective strategy for increasing the sensitivity and shortening the response/recovery time of semiconductor gas sensors used at room temperature with visible light illumination. However, the large band gap and the serious carrier recombination under light illumination of the semiconductor will hinder the improvement of room temperature sensing performance. Herein, we assembled the films of TiO2 nanoparticles decorated with polyoxometalate (POM) and organic dye molecules to enable accelerated room-temperature NO2 gas sensing (25 ℃) under visible light illumination. The POM molecule as electron acceptor in the dye/TiO2 films can enable the rapid separation and transport of photogenerated carriers. Combining with organic dyes, POM decorated dye/TiO2 films exhibited excellent sensing characteristics over a wide range of NO2 concentrations (50ppb-5 ppm), such as high sensitivity (233.1–1 ppm), relatively low detection concentration, and high selectivity. Moreover, the response/recovery time of NO2 from the surface of this sensitive films could be constantly controlled down to 48 s and 66 s, respectively, without any thermal energy.

Trapping of CO, CO2, H2S, NH3, NO, NO2, and SO2 by polyoxometalate compound

The forthcoming research includes the efficacy of polyoxometalate (POM) compound as an adsorbent for the following pollutant gases CO, CO2, H2S, NH3, NO, NO2, and SO2. The investigated complex was first optimized using TPSSh/LANL2DZ model chemistry to obtain the most stable geometry of the isolated and complex structures. After which the adsorption energy of the complex was ascertained in other to identify the complex with comparably stronger adsorption energy and the most stable complex. To understand the nature of intermolecular interactions occurring during the adsorption process, we carried out quantum theory of atoms in molecules (QTAIM) and natural bonding orbitals (NBO) calculations so as to carefully estimate the bond order of the studied complexes. It is very important to know the nature of conductivity of the complex being adsorbed as such this information is provided by the HOMO-LUMO energy gap. From energy gap considerations, there is increase as well as a corresponding decrease following the adsorption of each gas on polyoxometalate. The energy gap of the POM before adsorption was observed to be 0.602 eV, however upon adsorption of CO and CO2 it was observed to increase to 0.608 eV and 0.618 eV, with a difference of 0.006 eV and 0.016 eV. The increase in band gap was accompanied with a sharp decrease as H2S gas was adsorbed on POM with band gap of 0.122 eV a reduction of 0.496 eV from the band gap of CO2@POM, which indicate an increase in adsorption rate with a less stability of the adsorption process. The overall consideration of the adsorption energy and band gap shows that POM adsorbed NO, NH3 (NO@POM, NH3@POM) better since it had the highest adsorption energy; thus, NO and NH3 would be more efficiently detected by polyoxometalate molecule.

Single‐Molecule Confinement Induced Intrinsic Multi‐Electron Redox‐Activity to Enhance Supercapacitor Performance

Aggregation of polyoxometalates (POM) is largely responsible for the reduced performance of POM‐based energy‐storage systems. To address this challenge, here, the precise confinement of single Keggin‐type POM molecule in a porous carbon (PC) of unimodal super‐micropore (micro‐PC) is realized. Such precise single‐molecule confinement enables sufficient activity center exposure and maximum electron‐transfer from micro‐PC to POM, which well stabilizes the electron‐accepting molecules and thoroughly activates its inherent multi‐electron redox‐activity. In particular, the redox‐activities and electron‐accepting properties of the confined POM molecule are revealed to be super‐micropore pore size‐dependent by experiment and spectroscopy as well as theoretical calculation. Meanwhile, the molecularly dispersed POM molecules confined steadily in the “cage” of micro‐PC exhibit unprecedented large‐negative‐potential stability and multiple‐peak redox‐activity at an ultra‐low loading of ~11.4 wt%. As a result, the fabricated solid‐state supercapacitor achieves a remarkable areal capacitance, ultrahigh energy and power density of 443 mF cm−2, 0.12 mWh cm−2 and 21.1 mW cm−2, respectively. This work establishes a novel strategy for the precise confinement of single POM molecule, providing a versatile approach to inducing the intrinsic activity of POMs for advanced energy‐storage systems.

Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide.

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate-based zeolitic octahedral metal oxides have been synthesized. 31 P NMR spectroscopy shows that reduction of MoVI -based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na+ is used as the adsorbent, which effectively separates C2 hydrocarbon mixtures.

Physical Implementation of Reservoir Computing through Electrochemical Reaction (Adv. Sci. 6/2022)

Electrochemical Computing System In article number 2104076, Megumi Akai-Kasaya and co-workers develop a simple computing system based on the ionic current of electrochemical solutions at micro-scale electrodes. In this system, even deionized water can show great computational power in solving higher-order nonlinear problems. The addition of polyoxometalate molecules diversifies the Faradic currents, which makes it better at predicting periodic signals.

Physical Implementation of Reservoir Computing through Electrochemical Reaction.

Nonlinear dynamical systems serving reservoir computing enrich the physical implementation of computing systems. A method for building physical reservoirs from electrochemical reactions is provided, and the potential of chemical dynamics as computing resources is shown. The essence of signal processing in such systems includes various degrees of ionic currents which pass through the solution as well as the electrochemical current detected based on a multiway data acquisition system to achieve switchable and parallel testing. The results show that they have respective advantages in periodic signals and temporal dynamic signals. Polyoxometalate molecule in the solution increases the diversity of the response current and thus improves their abilities to predict periodic signals. Conversely, distilled water exhibits great computing power in solving a second‐order nonlinear problem. It is expected that these results will lead to further exploration of ionic conductance as a nonlinear dynamical system and provide more support for novel devices as computing resources.

Oxidation of phthalate acid esters using hydrogen peroxide and polyoxometalate/graphene hybrids

Phthalate acid esters (PAEs) have been adsorbed and oxidatively degraded into small molecules including lactic acid (LA), formic acid (FA), H2O and CO2 using polyoxometalates (POMs)/graphene hybrids. We demonstrated that super-lower concentrations of PAEs could be oxidized, which was due to their unique structure. POM molecules have been embedded onto graphene to form H5PMo10V2O40@surfactant(n)/Graphene(L wt%) (abbreviated as HPMoV@Surf(n)/GO(L wt%)) using surfactants with the carbon chain length n = 2, 4, 6 and 8 for the loading of HPMoV. The coexistence of the graphene and surfactant layer (on HPMoV@Surf(n)/GO(20 wt%)) adsorbed PAE molecules and transported them rapidly to HPMoV active sites. And n values determined the electron transfer ability between graphene and POMs that promoted PAEs oxidation. The loading of POMs on the surface of graphene permitted HPMoV@Surf(n)/GO(L wt%) act as interfacial catalyst which degraded various PAEs (i.e., diethyl phthalate (DEP), diallyl phthalate (DAP) and di (2-ethylhexyl) phthalate (DEHP)) while removed more than 70% of TOC and COD. The degradation of DEP achieved 93.0% with HPMoV@Surf(n)/GO(20 wt%) and H2O2, which followed first-order kinetics and the reaction activation energy (Ea) of 23.1 kJ/mol. Further, HPMoV@Surf(n)/GO(20 wt%) showed potential for the removal of PAEs in Wastewater Treatment Plant (WWTP), and the degradation efficiency for PAE (DEP) in secondary effluent achieved 55.0%. In addition, the loading method for POMs on graphene eliminated the leaching of POMs from graphene, and the degradation efficiency could still reach 88.1% after ten recycles.

Polymer Micelle Directed Magnetic Cargo Assemblies Towards Spin‐wave Manipulation

AbstractSpin‐wave based technologies that use collective oscillation of electrons termed magnons have been proposed for future computing landscapes due to their low energy consumption and high data transfer speeds. Magnonic crystals, materials with magnetic properties periodically varied in space, are central to such technologies. However, they are currently limited by the lithography techniques used for the magnetic patterning. To address this issue, bottom‐up self‐assembly using polymer templates to order magnetic cargo is presented. In this work, block copolymer micelles are used as templates to direct the organization of polyoxometalate (POM) molecules into organized assemblies. The structural organization of these assemblies is evaluated using microscopy and scattering techniques. The organized POM assemblies are demonstrated to modulate spin‐waves excited in permalloy thin films. This work demonstrates the first use of a bottom‐up approach to realize the fabrication of a magnonic assembly at the nanoscale. It further paves the way to achieve magnon‐mediated self‐assembled computing architectures.

Crosslinking ABA-type elastomers with polyoxometalate: A convenient molecular design of double network

In this study, we reported a convenient pathway toward the molecular design of double network materials. First, we prepared an ABA-type copolymer composed of a central poly (n-butyl acrylate) block and two 4-vinylpyridine end moieties, i.e., 4VP-PnBA-4VP. Since the central PnBA block was much larger than the two end 4VP blocks, sphere 4VP domains were formed in the PnBA matrix after the phase segregation. Second, we introduced a small number of polyoxometalate (POM) molecules that preferentially locate in the 4VP domain to cross-link only a fraction of the end moieties selectively. Since the cross-linked ends have an additional association energy, the sample exhibits two-step relaxation behavior. The amplitude and relaxation time of the slower process, owing to those chains end-cross-linked by POM, can be well explained by assuming (1) the reactivity is the same for all chain ends during the synthesize of the the end moiety, and (2) the probability for crosslinking by POM is the same for all 4VP monomers in their domain.

Single-molecule imaging and kinetic analysis of intermolecular polyoxometalate reactions.

We induce and study reactions of polyoxometalate (POM) molecules, [PW12O40]3− (Keggin) and [P2W18O62]6− (Wells–Dawson), at the single-molecule level. Several identical carbon nanotubes aligned side by side within a bundle provided a platform for spatiotemporally resolved imaging of ca. 100 molecules encapsulated within the nanotubes by transmission electron microscopy (TEM). Due to the entrapment of POM molecules their proximity to one another is effectively controlled, limiting molecular motion in two dimensions but leaving the third dimension available for intermolecular reactions between pairs of neighbouring molecules. By coupling the information gained from high resolution structural and kinetics experiments via the variation of key imaging parameters in the TEM, we shed light on the reaction mechanism. The dissociation of W–O bonds, a key initial step of POM reactions, is revealed to be reversible by the kinetic analysis, followed by an irreversible bonding of POM molecules to their nearest neighbours, leading to a continuous tungsten oxide nanowire, which subsequently transforms into amorphous tungsten-rich clusters due to progressive loss of oxygen atoms. The overall intermolecular reaction can therefore be described as a step-wise reductive polycondensation of POM molecules, via an intermediate state of an oxide nanowire. Kinetic analysis enabled by controlled variation of the electron flux in TEM revealed the reaction to be highly flux-dependent, which leads to reaction rates too fast to follow under the standard TEM imaging conditions. Although this presents a challenge for traditional structural characterisation of POM molecules, we harness this effect by controlling the conditions around the molecules and tuning the imaging parameters in TEM, which combined with theoretical modelling and image simulation, can shed light on the atomistic mechanisms of the reactions of POMs. This approach, based on the direct space and real time chemical reaction analysis by TEM, adds a new method to the arsenal of single-molecule kinetics techniques.