Self-assembly And Properties Of Carbonized Humic Acid Film And Graphene Based Composites

With the development of nanosciance and nanotechnology, the properties ofnanostructure materials were controlled by their composition, structure andmorphology. These functional nanomaterials can have properties that differ fromthose of the same bulk materials and can be used in certain circumstances. Thecomposites obtained from the self-assembly of nanomaterials display properties thatare different to those displayed by the individual nanoparticles and bulk materials.Several nanostructure materials, namely, graphene, silver nanoparticles, iron oxideand humic acid nanoparticles, were prepared to pave the way for variousself-assembly. These nanostructure materials were self-assembled into compositesand films to study the relationship of their properties and the morphology, structure.Furthermore, with the integration of green chemistry principles into the synthesis ofnanostructure materials, the goals, such as, the low cost of raw materials, the greenexperiment, the process of universality and controllability, as well as materialmorphology controllability, were sought achieve with efforts. The main contents andresults are summarized as follows:1. Multilayer films consisting of negatively charged humic acid and positivelycharged polyelectrolyte have been fabricated on various substrates using thelayer-by-layer self-assembly technique. The thickness （linearly increasing with thesquare root of NaCl concentration） and refractive index of the films determined byellipsometry can be regulated by ionic strength through adjusting the coiling of thepolyelectrolyte chains for assembly. The cone-shaped features on surface obtainedby atomic force microscope are derived from the negatively charged colloidal humicacid binding with polyelectrolyte cation. The smooth features are corresponding to the dissociated humic acid with carboxylate ion （―COO） electrostatically attractedon polyelectrolyte cation. These results are verified by Fourier transform infraredspectra. The linear dependence of the peak current on the square root of the scan raterevealed by the cyclic voltammetry indicates that the redox process at the electrodesurface is diffusion-limited and the charge transport does not involve the film itself.2. The aqueous graphene dispersions can be readily formed by reduction ofgraphene oxide colloids modified with ammonia. The dispersion can be stable overone month without the need for either polymer or surfactant stabilizers. Uniformfilms with thickness ranging from dozens of nanometers to more than a dozenmicrometers were prepared by flow directed self-assembly method and transferredto various substrates. As comparison to graphene oxide film, graphene film displaysa shiny metallic luster, high reflectivity and improved electrochemical properties.This graphene film electrode shows a specific electrochemical capacitance of121F/g, and is particularly promising for flexible supercapacitors. The electron transferreaction at the graphene film/solution interface is faster than that at the grapheneoxide film/solution interface using the Fe（CN）₆^3-/4-redox system due to therelationship of electrochemical properties and structure.3. A solution-based chemical approach has been used to prepare silvernanoparticle–graphene （Ag–G） composites through sequential reduction of grapheneoxidation and silver ions. The products can readily form a stable aqueous solutionwithout polymeric or surfactant stabilizers, and this makes it possible to producegraphene–silver composites on a large scale using low-cost solution processingtechniques. The paper-like Ag–G film obtained by vacuum filtration is glossy andhas a high reflectivity and good flexibility. Raman signals of graphene for the filmare increased from three-to eightfold with the increase of silver nanoparticle contentin the composites, indicating that the increase can be tuned by changing the densityof silver nanoparticles. The electrochemical properties of the Ag–G film demonstratetheir fast electron-transfer kinetics for the Fe（CN）₆^3-/4-redox system.4. Monodisperse iron oxide nanoparticle-reduced graphene oxide compositeswere prepared by self-assembly in aqueous phase. Pre-synthesized iron oxide nanoparticles were transferred to aqueous phase and then mixed with grapheneoxide dispersion. The iron oxide nanoparticles, with diameter of around10nm, wereself-assembled on the reduced graphene oxide sheets through electrostatic attractionduring the reduction of graphene oxide. X-ray diffraction and selected area electrondiffraction indicated the inverse spinel structure of the iron oxide nanoparticles. Themagnetization curves indicated that all samples exhibited ferromagnetic behavior atroom temperature with small coercivity. The values of specific saturatedmagnetization of composites with different densities of iron oxide nanoparticles are38.3,19.5and7.7emu/g. Permittivity and permeability of composites exhibitobvious fluctuations, which are ascribed to the multiple magnetic resonances. Themultiple resonances involve exchange resonances （the consequence of small sizeeffect, surface effect and spin wave excitations） and natural resonance.5. We describe a simple and green method for preparing silver nanoparticleswith an average diameter in the range from4.0nm to5.0nm and with the standarddeviation as low as0.3nm （polydispersity:5.5%）. The pre-synthesized silver oleateprecursor in oleic acid was thermally reduced to obtain the monodisperse silvernanoparticles without a size selection procedure. This environmentally friendlychemistry approach requires only three reagents （silver nitrate, sodium oleate andoleic acid） which all come from renewable resources. The average diameter of thesenanoparticles can be greatly adjusted by reaction time and finely regulated byreaction temperature at same precursor concentration. The nucleation/diffusionalgrowth model can be used to interpret the size and morphology difference of silvernanoparticles synthesized in different condition. Raman spectra of graphene withand without an active layer of silver nanoparticles were measured to investigate thesurface-enhanced Raman scattering properties of silver nanoparticles. Raman signalof graphene with an active layer increased by fifteen-fold in comparison to thatwithout an active layer. The surface-enhanced Raman scattering activity can beexplained using the interparticle coupling induced Raman enhancement.