Haolong Li

Layer-by-Layer Assembly and UV Photoreduction of Graphene–Polyoxometalate Composite Films for Electronics

Graphene oxide (GO) nanosheets and polyoxometalate clusters, H(3)PW(12)O(40) (PW), were co-assembled into multilayer films via electrostatic layer-by-layer assembly. Under UV irradiation, a photoreduction reaction took place in the films which converted GO to reduced GO (rGO) due to the photocatalytic activity of PW clusters. By this means, uniform and large-area composite films based on rGO were fabricated with precisely controlled thickness on various substrates such as quartz, silicon, and plastic supports. We further fabricated field effect transistors based on the composite films, which exhibited typical ambipolar features and good transport properties for both holes and electrons. The on/off ratios and the charge carrier mobilities of the transistors depend on the number of deposited layers and can be controlled easily. Furthermore, we used photomasks to produce conductive patterns of rGO domains on the films, which served as efficient microelectrodes for photodetector devices.

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.

Electrochemical‐Reduction‐Assisted Assembly of a Polyoxometalate/Graphene Nanocomposite and Its Enhanced Lithium‐Storage Performance

Herein, we present an electrochemically assisted method for the reduction of graphene oxide (GO) and the assembly of polyoxometalate clusters on the reduced GO (rGO) nanosheets for the preparation of nanocomposites. In this method, the Keggin-type H4 SiW12O40 (SiW12) is used as an electrocatalyst. During the reduction process, SiW12 transfers the electrons from the electrode to GO, leading to a deep reduction of GO in which the content of oxygen-containing groups is decreased to around 5%. Meanwhile, the strong adsorption effect between the SiW12 clusters and rGO nanosheets induces the spontaneous assembly of SiW12 on rGO in a uniformly dispersed state, forming a porous, powder-type nanocomposite. More importantly, the nanocomposite shows an enhanced capacity of 275 mAh g(-1) as a cathode active material for lithium storage, which is 1.7 times that of the pure SiW12. This enhancement is attributed to the synergistic effect of the conductive rGO support and the well-dispersed state of the SiW12 clusters, which facilitate the electron transfer and lithium-ion diffusion, respectively. Considering the facile, mild, and environmentally benign features of this method, it is reasonable as a general route for the incorporation of more types of functional polyoxometalates onto graphene matrices; this may allow the creation of nanocomposites for versatile applications, for example, in the fields of catalysis, electronics, and energy storage.

Self‐Assembly and Structural Evolvement of Polyoxometalate‐Anchored Dendron Complexes

A cationic dendritic molecule that has alkyl chains has been synthesized and employed to encapsulate anionic polyoxometalates through electrostatic interactions. The prepared surfactant-encapsulated polyoxometalate (SEP) complexes were used as building blocks to fabricate self-assemblies in solution and the solid state. Monodispersion, lamellar, and columnar assemblies of SEP complexes have been characterized in detail. With increasing the number of peripheral cationic dendrons on inorganic clusters, the SEPs undergo changes from globular assemblies to monodispersions in solution and from lamellar assemblies to hexagonal columnar structures in the solid state, depending on the amounts of cationic dendrons in the complexes. The structural evolvement was simulated through consideration of the size and shape of the cationic dendron and polyanionic clusters, and the experimental results are in good agreement with the interpretation of the simulations. The present research demonstrates a new kind of dendritic complex and provides a route for controlling their assembling states by simply alternating the number of cationic dendrons in the complexes.

Chiral heteropoly blues and controllable switching of achiral polyoxometalate clusters.

Managing the blues: Chiral heteropoly blues of achiral polyoxometalate clusters were created through an intermolecular interaction with a chiral organic compound. Controllable chiroptical switching of the cluster complexes was possible through reversible photochromism of the polyoxometalates (see picture).

Assembly of Cerium(III)-Stabilized Polyoxotungstate Nanoclusters with SeO32−/TeO32− Templates: From Single Polyoxoanions to Inorganic Hollow Spheres in Dilute Solution

A versatile one-pot strategy was employed to synthesize three cerium(III)-stabilized polyoxotungstates nanoclusters by combining cerium linkers and SeO3(2-)/TeO3(2-) heteroanion templates: K32Na16[{(XO3)W10O34}8{Ce8(H2O)20}(WO2)4(W4O12)]·nH2O [X=Se, n=81 (1); X=Te, n=114 (2)] and K12Na22[{(SeO3)W10O34}8{Ce8(H2O)20}(WO2)4{(W4O6)Ce4(H2O)14(SeO3)4(NO3)2}]· 79H2O (3), which are the first lanthanide-containing polyoxotungstates with selenium or tellurium heteroatoms. The three clusters were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric/differential thermal analysis, UV/Vis spectroscopy, ESI-MS, and X-ray photoelectron spectroscopy. Their electrochemical, photoluminescence, and magnetic properties were investigated. Their behavior in solution was studied by transmission electron microscopy, which showed that their single polyoxoanions assemble into intact, uniform-sized, purely inorganic hollow spheres in dilute water/acetone solution.

Supramolecular assembly of chiral polyoxometalate complexes for asymmetric catalytic oxidation of thioethers

In this paper, a chiral amphiphilic cation with two stereocenters has been employed to encapsulate a well-known catalytically activated sandwich-type polyoxometalate, Na12[WZn3(H2O)2(ZnW9O34)2], through electrostatic interaction. The prepared chiral organic cation-encapsulated polyoxometalate complexes were found to self-aggregate into spherical supramolecular assemblies with a diameter of ca. 100 nm in the reaction solution. These assemblies, serving as microreactors, exhibited an efficient asymmetric catalytic activity for the oxidation of sulfide with up to 72% enantiomeric excess. The coverage density of chiral organic cations enwrapped on the polyoxometalate surface was proved to have an important influence on the enantioselectivity. Detailed kinetic study of the catalytic process showed that the enantioselectivity of sulfide oxidation was derived from the combination of an ineffective asymmetric sulfoxidation and an effective kinetic resolution of the sulfoxide. The present research strategically offers an understanding for the direct and efficient construction of polyoxometalate based supramolecular catalysts for asymmetric reactions through controlling the surface microenvironment.

Controllable vesicular structure and reversal of a surfactant-encapsulated polyoxometalate complex

An organic–inorganic complex, surfactant-encapsulated polyoxometalate (DDDA)9EuW10O36, demonstrates reversible self-assembly behavior in organic solvents and water. This hybrid complex can spontaneously organize into inverse vesicles by simply dispersing it in an organic solvent. Interestingly, by dissolving the water-insoluble complex in a water-miscible organic solvent such as ethanol and subsequently addition of water, it could be transferred into aqueous solution and the inverse vesicles in the organic solvent transformed into a regular bilayer structure in water. The vesicular aggregate, which had a regular structure, was studied by dynamic light scattering and transmission electron microscopy, as well as X-ray diffraction. The structural transformation was proved by zeta potential analysis and X-ray photoelectron spectroscopy, and the process was followed by 1H NMR. These results provide the first example of aggregation behavior in this kind of complex, which is different from that exhibited by well known amphiphilic molecules with polar and non-polar ends, in water. Moreover, the reverse process, from a regular bilayer to inverse vesicles, can be conveniently carried out by simply extracting the complex from water into the organic phase. The results described may provide new opportunities in performing catalytic and biomimetic functions of polyoxometalates.

Polyoxometalate-modulated self-assembly of polystyrene-block-poly(4-vinylpyridine)

Polyoxometalates can serve as active components to induce and modulate the micellization behavior of polystyrene-block-poly(4-vinylpyridine).

Instantaneous and reversible gelation of organically grafted polyoxometalate complexes with dicarboxylic acids

An organically pyridyl-grafted Anderson-type POM complex (TBA–Py–MnMo6) bearing counterions of tetrabutylammonium (TBA) was found to form supramolecular gels instantaneously through mixing with appropriate dicarboxylic acid additives in acetonitrile. In the gelation process, TBA–Py–MnMo6 was confirmed to self-assemble into supramolecular polymer chains through unidirectional hydrogen bonding between the pyridyl groups of POMs and the carboxylic groups of dicarboxylic acids, which drove the complexes into one-dimensional fibers. The fibers were demonstrated to further intertwine together through the lateral hydrogen bonding between polymer chains under the support of excess acid, forming three-dimensional networks. The distance between TBA–Py–MnMo6 units secluded by dicarboxylic acid was proved to be a criterion for the formation of gels, which is related to the TBA density along the supramolecular polymer chains and the solvent–fiber interfacial energy. The gelation behaviour of the hybrid POM complex can be simply controlled through adjusting the length of dicarboxylic acids. More interestingly, the POM hybrid gels exhibited a quick response to organic bases and diacids reversibly.