Soft Matter Approaches for Enhancing the Catalytic Capabilities of Polyoxometalate Clusters

In this minireview, we discuss the recent efforts on expanding the catalytic capabilities of polyoxometalates (POM) through emulsion catalysis approaches with novel catalytic-active POM–organic hybrid clusters as emulsifiers. The hybrid emulsifiers include surfactant encapsulated POM complexes, molecular POMs–organic hybrids, and POM-based solid nanoparticles. With such novel approaches the catalytic efficiency of the POMs can be significantly improved by enhancing the compatibility of the POMs with organic media, providing catalytic interface for biphasic reactions, as well as easier preparation, and better recyclability. Particularly, a simple, green chemistry method to prepare metal nanoparticle materials with POMs as both reducing and capping agents in aqueous is reviewed.     

新型Ti3C2 MXene的化学制备及基于MXene忆阻器特性与机理

阻变器件是一种微电子器件,具有阻值可在两个甚至两个以上的阻态之间重复变化的特点。忆阻器作为新型的阻变器件,具有可连续变化的丰富阻态。近年来因其具备简单的二端结构、高集成度以及低工作电压等特性,在新型非易失性存储以及构建神经形态系统等方面被广泛研究。但其在实现应用的过程中仍存在着稳定性较差等问题。近期一些工作证明了二维材料如氧化石墨烯在优化忆阻器性能方面具备良好的应用潜力。MXene是一种具备类似石墨烯结构的新型二维过渡金属碳/氮化物,因其具备二维层状结构显现出特殊的力学以及电学特性,有望应用于忆阻器中以提高器件的电学性能。在本文中,我们通过化学湿法刻蚀制备了Ti₃C₂粉末,通过旋涂工艺在忆阻器结构中引入Ti₃C₂薄膜。Ti₃C₂ MXene与SiO₂同时作为忆阻器阻变层,制备了Cu/Ti₃C₂/SiO₂/W结构的忆阻器,并且对其相关电学特性进行了探究。在该器件上,通过实验测得忆阻器典型的开关特性曲线并在双向直流电压下针对高、低阻态的可重复性、稳定性进行了实验。结果表明该器件能够在100个扫描循环过程中保持稳定的高、低阻态达到10⁴ s以上。同时,该器件状态能够受脉冲电压调节,实现突触间典型的双脉冲易化行为。实验结果表明基于Ti₃C₂ MXene的忆阻器将有望应用于构建新兴存储设备以及人工神经形态系统。     

Metal–Organic Frameworks–Based Memristors: Materials,Devices, and Applications

Facing the explosive growth of data, a number of new micro-nano devices with simple structure, low power consumption, and size scalability have emerged in recent years, such as neuromorphic computing based on memristor. The selection of resistive switching layer materials is extremely important for fabricating of high performance memristors. As an organic-inorganic hybrid material, metal-organic frameworks (MOFs) have the advantages of both inorganic and organic materials, which makes the memristors using it as a resistive switching layer show the characteristics of fast erasing speed, outstanding cycling stability, conspicuous mechanical flexibility, good biocompatibility, etc. Herein, the recent advances of MOFs-based memristors in materials, devices, and applications are summarized, especially the potential applications of MOFs-based memristors in data storage and neuromorphic computing. There also are discussions and analyses of the challenges of the current research to provide valuable insights for the development of MOFs-based memristors.     

Nanoscale polyoxometalate‐based inorganic/organic hybrids

The latest advances in the area of polyoxometalate (POM)‐based inorganic/organic hybrid materials prepared by self‐assembly, covalent modification, and supramolecular interactions are presented. This Review is composed of five sections and documents the effect of organic cations on the formation of novel POMs, surfactant encapsulated POM‐based hybrids, polymeric POM/organic hybrid materials, POMs‐containing ionic crystals, and covalently functionalized POMs. In addition to their role in the charge‐balancing, of anionic POMs, the crucial role of organic cations in the formation and functionalization of POM‐based hybrid materials is discussed. DOI 10.1002/tcr.201100002     

Polyoxometalates as Potential Next‐Generation Metallodrugs in the Combat Against Cancer

Polyoxometalates (POMs) are an emerging class of inorganic metal oxides, which over the last decades demonstrated promising biological activities by the virtue of their great diversity in structures and properties. They possess high potential for the inhibition of various tumor types; however, their unspecific interactions with biomolecules and toxicity impede their clinical usage. The current focus of the field of biologically active POMs lies on organically functionalized and POM‐based nanocomposite structures as these hybrids show enhanced anticancer activity and significantly reduced toxicity towards normal cells in comparison to unmodified POMs. Although the antitumor activity of POMs is well documented, their mechanisms of action are still not well understood. In this Review, an overview is given of the cytotoxic effects of POMs with a special focus on POM‐based hybrid and nanocomposite structures. Furthermore, we aim to provide proposed mode of actions and to identify molecular targets. POMs are expected to develop into the next generation of anticancer drugs that selectively target cancer cells while sparing healthy cells.     

MoTe2-based low energy consumption artificial synapse for neuromorphic behavior and decimal arithmetic

Two-dimensional material-based memristors have shown attractive research prospects as brain-like devices for neuromorphic computing. Among them, transition metal dichalcogenides–based memristors have proved to be one of the most promising competitors. In this work, a two-dimensional memristor based on MoTe₂ nanosheets was fabricated and demonstrated. The experimental results illustrate that the two-terminal synaptic based on the Ag/MoTe₂/ITO structure exhibits stable bipolar and non-volatile resistive switching characteristics attributed to the controllable formation and rupturing of silver conductive filaments. The device can be successively modulated by a pulse train with a minimum pulse width of 40 ns. More interestingly, the energy consumption of the device to complete one write event is only 74.2 pJ. In addition, biological synaptic behaviors, such as excitatory postsynaptic current gain properties, long-term potentiation (LTP)/long-term depression, spike-timing-dependent- plasticity, short-term plasticity, long-term potentiation (LTP), paired-pulse facilitation, post-tetanic potentiation, and learning-experimental behaviors were mimicked faithfully. Finally, the decimal arithmetic application was introduced to the device, and it is confirmed that addition and multiplication functions can be performed. Therefore, the artificial synapse based on MoTe₂ nanosheets not only exhibits the stable non-volatile resistive switching behavior but also facilitates the development of low-energy consumption neuromorphic computing chips based on transition metal dichalcogenides.     

Organic Memristor Based on High Planar Cyanostilbene/Polymer Composite Films

Organic memristors with low power consumption, fast write/erasure speed, and complementary metal-oxide-semiconductor(CMOS) compatibility have attracted tremendous attention to mimic biological synapses to realize neuromorphic computation in recent years. In this paper, organic resistive switching memory(ORSM) based on (Z)-3-(naphthalen-2-yl)-2-(4-nitrophenyl)acrylonitrile(NNA) and polymer poly(N-vinylcarbazole)(PVK) composite film was prepared by spin-coating method. Device performance based on NNA:PVK composite films with different mass fractions of NNA were systematically investigated. The ORSM based on PVK:40%(mass fraction) NNA composite film exhibited non-volatile and bipolar memory properties with a switching ratio(I_on/I_off) of 24.1, endurance of 68 times and retention time of 10⁴ s, a “SET” voltage(V_set) of -0.55 V and a “RESET” voltage(V_reset) of 2.35 V. The resistive switching was ascribed to the filling and vacant process of the charge traps induced by NNA and the inherent traps in PVK bulk. The holes trapping and de-trapping process occurred when the device was applied with a negative or positive bias, which caused the transforming of the conductive way of charges, that is the resistive behaviors in the macroscopic. This study provides a promising platform for the fabrication of ORSM with high performance.     

Transition metal ion-substituted polyoxometalates entrapped in polypyrrole as an electrochemical sensor for hydrogen peroxide

A conducting polymer was used for the immobilization of various transition metal ion-substituted Dawson-type polyoxometalates (POMs) onto glassy carbon electrodes. Voltammetric responses of films of different thicknesses were stable within the pH domain 2-7 and reveal redox processes associated with the conducting polymer, the entrapped POMs and incorporated metal ions. The resulting POM doped polypyrrole films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. An amperometric sensor for hydrogen peroxide detection based upon the POM doped polymer films was investigated. The detection limits were 0.3 and 0.6 μM, for the Cu(2+)- and Fe(3+)-substituted POM-doped polypyrrole films respectively, with a linear region from 0.1 up to 2 mM H(2)O(2). Surface characterization of the polymer films was carried out using atomic force microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy.     

Tailor–made Covalent Organic‐Inorganic Polyoxometalate Hybrids: Versatile Platforms for the Elaboration of Functional Molecular Architectures

Post‐functionalization of organically modified polyoxometalates (POMs) is a powerful synthetic tool to devise functional building blocks for the rational elaboration of POM‐based molecular materials. In this personal account we focus on iodoaryl‐terminated POM platforms, describe reliable routes to the synthesis of covalent organic‐inorganic POM‐based hybrids and their integration into advanced molecular architectures or multi‐scale assemblies as well as their immobilization onto surfaces. Valorisation of the remarkable redox properties of POMs in the fields of artificial synthesis and molecular electronic is especially considered.     

Stable photochromism and controllable reduction properties of surfactant-encapsulated polyoxometalate/silica hybrid films

In this paper, we present a novel strategy for fabricating polyoxometalate (POM)-based photochromic silica hybrid films. To combine metal nanoparticles (NPs) into the POMs embedded silica matrix, furthermore, we realized the controllable in situ synthesis of metal NPs in the film by utilizing the reduction property of POMs existing in the reduced state. Through electrostatic encapsulation with hydroxyl-terminated surfactants, the POMs with good redox property can be covalently grafted onto a silica matrix by means of a sol-gel approach, and stable silica sol-gel thin films containing surfactant-encapsulated POMs can be obtained. The functional hybrid film exhibits both the transparent and easily processible properties of silica matrix and the stable and reversible photochromism of POMs. In addition, well-dispersed POMs in a hydrophobic microenvironment within the hybrid film can be used as reductants for the in situ synthesis of metal NPs. More significantly, the size and location of NPs can be tuned by controlling the adsorption time of metal ions and mask blocking the surface. The hybrid film containing both POMs and metal NPs with patterned morphology can be obtained, which has potential applications in optical display, memory, catalysis, microelectronic devices and antibacterial materials.     

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal‐oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self‐assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM–cation interactions in solution, the resulting solid‐state compounds, and behavior and properties that emerge from these POM–cation interactions. We will explore how application‐inspired research has exploited cation‐controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM–cation interactions.     

Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal–oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.     

Molecular Design of Polyoxometalate-Based Compounds for Environmentally-Friendly Functional Group Transformations: From Molecular Catalysts to Heterogeneous Catalysts

This review article summarizes our recent researches for molecular design of polyoxometalates (POMs) and their related compounds for environmentally-friendly functional group transformations. The divacant POM [γ-SiW₁₀O₃₄(H₂O)₂]⁴⁻ exhibits high catalytic performance for mono-oxygenation-type reactions including epoxidation of olefins and allylic alcohols, sulfoxidation, and hydroxylation of organosilanes with H₂O₂. We have successfully synthesized several POM-based molecular catalysts (metal-substituted POMs) with controlled active sites by the introduction of metal species into the divacant POM as a “structural motif”. These molecular catalysts can efficiently activate H₂O₂ (vanadium-substituted POM for epoxidation) and alkynes (copper-substituted POM for click reaction and oxidative homocoupling of alkynes). The aluminum-substituted POM exhibits Lewis acidic catalysis for diastereoselective cyclization of (+)-citronellal to (−)-isopulegol. In addition, we have developed POM-based “molecular heterogeneous catalysts” by the “solidification” and “immobilization” of catalytically active POMs.     

New Anderson-Based Polyoxometalate Covalent Organic Frameworks as Electrodes for Energy Storage Boosted Through Keto-Enol Tautomerization

The unique electrochemical properties of polyoxometalates (POMs) render them ideal components for the fabrication of next-generation high-performance energy storage systems. However, their practical applications have been hindered by their high solubility in common electrolytes. This problem can be overcome by the effective hybridization of POMs with other materials. Here we present the design and synthesis of two novel polyoxometalate-covalent organic frameworks (POCOF) via one-pot solvothermal strategy between an amino-functionalized Anderson-type POM and a trialdehyde-based building unit. We show that structural and functional complexity can be enriched by adding hydroxyl groups in the 2,4,6 position to the benzene-1,3,5-tricarbaldehyde allowing to exploit for the first time in POCOFs the keto-enol tautomerization as additional feature to impart greater chemical stability to the COFs and enhanced properties leading to large specific surface area (347 m²/g) and superior electrochemical performance of the POCOF-1 electrodes, when compared with POCOF-2 electrodes that possess only imine-type linkage and with pristine POM electrodes. Specifically, POCOF-1 electrodes display remarkable specific, areal, and volumetric capacitance (125 F/g, 248 mF/cm² and 41.9 mF/cm³, respectively) at a current density of 0.5 A/g, a maximum energy density (56.2 Wh/kg), a maximum power density (3.7 kW/kg) and an outstanding cyclability (90 % capacitance retention after 5000 cycles).     

Polyoxometalate‐Mediated One‐Pot Synthesis of Pd Nanocrystals with Controlled Morphologies for Efficient Chemical and Electrochemical Catalysis

Polyoxometalates (POMs), as inorganic ligands, can endow metal nanocrystals (NCs) with unique reactivities on account of their characteristic redox properties. In the present work, we present a facile POM‐mediated one‐pot aqueous synthesis method for the production of single‐crystalline Pd NCs with controlled shapes and sizes. The POMs could function as both reducing and stabilizing agents in the formation of NCs, and thus gave a fine control over the nucleation and growth kinetics of NCs. The prepared POM‐stabilized Pd NCs exhibited excellent catalytic activity and stability for electrocatalytic (formic acid oxidation) and catalytic (Suzuki coupling) reactions compared to Pd NCs prepared without the POMs. This shows that the POMs play a pivotal role in determining the catalytic performance, as well as the growth, of NCs. We envision that the present approach can offer a convenient way to develop efficient NC‐based catalyst systems.     

The redox of hydroxyl-assisted metallic filament induced resistive switching memory based on a biomaterial-constructed sustainable and environment-friendly device

Using biomaterial-constructed resistive switching memory devices has attracted great attention for the potential application in advanced electronic components because of their variety of advantages, such as low cost, sustainability, environment-friendly, and so on. In this work, after a series of treatments, an edible mushroom as an intermediate insulating material was assembled into an Ag/biofilm/metal structure, in which the metals such as Al, Cu, Ag, and Ti were chosen as the bottom electrode to explore in-depth physical mechanisms. The transmission mechanism of resistive switching memory behavior in an Ag/biofilm/metal device was studied in detail. It was found that the redox of hydroxyl-assisted Ag filaments could be easily formed on an inactive metal bottom electrode through the mushroom film by redox reaction under applied voltage. This work provides an ingenious potential application in wearable, flexible, sustainable, and environment-friendly biologically resistive random-access memories.     

Spatial nonuniformity in resistive-switching memory effects of NiO

Electrically driven resistance change phenomenon in metal/NiO/metal junctions, so-called resistive switching (RS), is a candidate for next-generation universal nonvolatile memories. However, the knowledge as to RS mechanisms is unfortunately far from comprehensive, especially the spatial switching location, which is crucial information to design reliable devices. In this communication, we demonstrate the identification of the spatial switching location of bipolar RS by introducing asymmetrically passivated planar NiO nanowire junctions. We have successfully identified that the bipolar RS in NiO occurs near the cathode rather than the anode. This trend can be interpreted in terms of an electrochemical redox model based on ion migration and p-type conduction.     

脉冲激光沉积制备非晶La0.75Sr0.25MnO3薄膜用于半透明阻变存储器

用脉冲激光沉积方法制备非晶La_0.75Sr_0.25MnO₃（a-LSMO）薄膜作为阻变器件（Ag/a-LSMO/ITO）的中间层,所得器件具有良好的非易失性和双极阻变行为。ITO衬底及超薄a-LSMO薄膜具有很高的可见光透过率,从而可制备半透明阻变器件。通过高分辨透射电镜直接观测到了在银电极与ITO电极间的银导电细丝。器件的阻变特性归因于在非晶镧锶锰氧层中的银导电细丝的生长与断裂。     

脉冲激光沉积制备非晶La0.75Sr0.25MnO3薄膜用于半透明阻变存储器

用脉冲激光沉积方法制备非晶La_0.75Sr_0.25MnO₃（a-LSMO）薄膜作为阻变器件（Ag/a-LSMO/ITO）的中间层,所得器件具有良好的非易失性和双极阻变行为。ITO衬底及超薄a-LSMO薄膜具有很高的可见光透过率,从而可制备半透明阻变器件。通过高分辨透射电镜直接观测到了在银电极与ITO电极间的银导电细丝。器件的阻变特性归因于在非晶镧锶锰氧层中的银导电细丝的生长与断裂。