Fabrications And Synergistic Functions Of Polyoxometalate/Graphene Composites

Polyoxometalates (POMs) are a class of single molecule clusters with structuraldiversities and show excellent properties in catalysis, optics, electron, magnetism and soon. However, their crystal or powder state and poor stablility sensitive to pH valuehinder POM’s practical applications. In order to overcome or avoid these shortcomings,POM-based materials have become a research hotspot in POM chemistry, which canimprove the processability, stabilities and functional performance of POMs due to thesynergistic effects of POMs and functional matrixes.Previously, the mostly used matrixes for POM-based materials were polymer, silica,hydrotalcite and carbon nanotube, but with the advent of graphene which is a newtwo-dimensional carbon nanomaterial with unique properties, POM/graphenecomposites have attracted widespread attentions. However, the development of thePOM/graphene composites still remains many challenges, for example, how toefficiently prepare or exploit novel functions of POM/graphene composites.It is known that POMs can stably adsorb on graphene nanosheets by the electrontransfer interaction between POM and graphene. In this dissertation, we used thisinteraction as a new versatile intermolecular force to assemble various POM/graphene composites through different methods. Meanwhile, we studied the synergistic functionalapplications of POM/graphene composites and their new features which wereintroduced by the functional cations.First, for commercial POMs (such as H4SiW12O40and H3PW12O40), based on theelectron transfer supramolecular interaction between POM and graphene, we developedan electrochemical-reduction method to assemble POM/graphene composites. In thismethod, POMs acted as electrocatalysts to transfer the electrons from electrode tographene oxide (GO), resulting in the deep reduction of GO to reduced GO (RGO),meanwhile POMs adsorbed on the RGO nanosheets through the electron transferinteraction, forming POM/RGO composites. By optimizing the conditions of thereduction potential, the concentration of H4SiW12O40(SiW) and the reduction time, wesuccessfully prepared a porous and powder-type SiW/RGO composite. HRTEM andXRD results reveal that the SiW clusters are uniformly dispersed on the RGOnanosheets, and we also verified the existence of the electron transfer interactionbetween SiW and RGO by means of XPS and FTIR spectra. In addition, due to thesynergistic effect of the conductive RGO and the good dispersion of SiW on RGO, theSiW/RGO composite as a cathode active material exhibit higher lithium-storageperformance (275mAhg1), which is1.7times of pure SiW. Taking the advantages offacile, environmentally friendly and suitable for mass production, and thegeneralizability for common POMs, it can be served as a general method to fabricatevarious functional POM/graphene composites for applications in catalysis, electronics,capacitance and sensors.Second, using cationic surfactants encapsulated POMs (SEPs) as clustersuprasurfactants, in view of the above verified electron transfer interaction, we proposeda new project for assembling SEP/RGO composites by phase transfer method. In SEP,the hydrophilic POM core and hydrophobic alkyl chains shell are connected bynon-direction Coulomb interaction, in which the alkyl chains can move in the Coulomb field around POM. Therefore, at the oil/water interface, the phase separation of SEPoccurs with hydrophilic POM toward water and hydrophobic alkyl chains towards oil,forming an amphiphilic structure. Under the assistance of the electron transferinteraction, the naked POM of SEP uniformly adsorbed on RGO nanosheets, while thehydrophobic alkyl chains of SEP pulled RGO to organic solvents, realizing the phasetransfer of RGO from water to low polar organic solvents and the fabrication of aSEP/RGO composite. According to the XPS and FTIR results, the electron transferinteraction between POM and graphene is not interfered by the introduction of thesurfactants. Moreover, due to the introduction of alkyl chains, the SEP/RGO compositeshows good single layer dispersibility in low polar organic solvents and intercalatedstructure in solid. This finding not only realizes the functionalization of graphene withinorganic clusters, but also provides a new approach for the dispersion of RGO in lowpolar organic solvents, which is meaningful for constructing functional graphenecomposites.Third, we used surfactants containing trithioester terminal group to encapsulateSiW, then grafted polystyrene (PS) chains for cluster-cored supramolecular starpolymers (CSPs) by reversible addition-fragmentation chain-transfer (RAFT)polymerization. Depending on the above electron transfer interaction and mechanism ofphase transfer method, CSP can also adsorb on RGO, and transfer RGO from water tooil phase to constructe CSP/RGO composites, realizing facile functionalization of RGOwith polymer. And the CSP/RGO composite can be dispersed as single layers in lowpolar organic solvents. Furthermore, upon simply altering the amount of CSP used forphase transfer of RGO, the grafting density of CSP on RGO nanosheets can becontrolled and due to the polyoxometalates with high electron density as grafting sites,visual observation of polymer-grafted sites on graphene can be realized. In addition,when the CSP/RGO composites were filled into PS by solution processing, themechanical property of the PS coatings were significantly enhanced. This phase transfer method for fabrication of the CSP/RGO composites can easily realize the controllablepolymer functionalization of graphene and the well-dispersion of graphene in low polarorganic solvent, laying a good foundation on its materials and devices and opening anew sight for the functionalization of graphene with polymer.In conclusion, in this thesis we use the electron transfer between POM andgraphene as a new general intermolecular force to fabricate various POM/graphenecomposites. With this force and electrochemical-reduction method, we assembledcommon POMs on RGO nanosheets and achieved the synergistic application of theobtained POM/graphene composites in energy storage. In addition, we took SEPs andCSPs as cluster suprasurfactants and used phase transfer method to realize themonolayer dispersion of graphene in low polar organic solvents, the controllablegrafting density and the visual grafting sites of polymer functionalized graphene. Theelectron transfer interaction, electrochemical reduction method and phase transfermethod are instructive for the fabrication of POM-based materials and devices.