Because of its high theoretical specific capacity (4200mAh/g) and natural abundance (2nd most abundant element on earth), silicon is considered a promising anode candidate for high energy density lithium-ion batteries. However, the dramatic volume changes (up to 400%) that occur during lithiation/delithiation and the relative low electrical conductivity of silicon prevent the implementation of this material. In this work, a nano-silicon/polyaniline/reduced graphene oxide composite was synthesized via a two-step process: in-situ polymerization of polyaniline (PANi) in the presence of nano-silicon followed by combination of the prepared n-Si/PANi binary composite with reduced graphene oxide (RGO), to form a n-Si/PANi/RGO composite. Electron microscopy reveals the unique nano-architecture of the n-Si/PANi/RGO composite: silicon nanoparticles are well dispersed within a PANi matrix, which in turn is anchored to the surface of RGO sheets. The n-Si/PANi/RGO ternary composite delivered an initial capacity of 3259 mAh/g and 83.5% Coulombic efficiency. The new composite displayed better rate performance and capacity recovery than either nano-Si or n-Si/PANi. Structural and morphological studies combined with AC impedance analysis suggest that the n-Si/PANi/RGO composite has higher electrical conductivity than the other two component materials, yielding better performance at high current densities or C rates. The good rate performance, high initial specific capacity and stable Coulombic efficiency of n-Si/PANi/RGO make it a promising anode material for high energy density lithium-ion batteries.

Because of its high theoretical specific capacity (4200mAh/g) and natural abundance (2nd most abundant element on earth), silicon is considered a promising anode candidate for high energy density lithium-ion batteries. However, the dramatic volume changes (up to 400%) that occur during lithiation/delithiation and the relative low electrical conductivity of silicon prevent the implementation of this material. In this work, a nano-silicon/polyaniline/reduced graphene oxide composite was synthesized via a two-step process: in-situ polymerization of polyaniline (PANi) in the presence of nano-silicon followed by combination of the prepared n-Si/PANi binary composite with reduced graphene oxide (RGO), to form a n-Si/PANi/RGO composite. Electron microscopy reveals the unique nano-architecture of the n-Si/PANi/RGO composite: silicon nanoparticles are well dispersed within a PANi matrix, which in turn is anchored to the surface of RGO sheets. The n-Si/PANi/RGO ternary composite delivered an initial capacity of 3259 mAh/g and 83.5% Coulombic efficiency. The new composite displayed better rate performance and capacity recovery than either nano-Si or n-Si/PANi. Structural and morphological studies combined with AC impedance analysis suggest that the n-Si/PANi/RGO composite has higher electrical conductivity than the other two component materials, yielding better performance at high current densities or C rates. The good rate performance, high initial specific capacity and stable Coulombic efficiency of n-Si/PANi/RGO make it a promising anode material for high energy density lithium-ion batteries.

Magister Scientiae - MSc / It is well known that a lead pencil is made of graphite, a naturally form of carbon, this is important but not very exciting. The exciting part is that graphite contains stacked layers of graphene and each and every layer is one atom thick. Scientists believed that these graphene layers could not be isolated from graphite because they were thought to be thermodynamically unstable on their own and taking them out from the parent graphite crystal will lead them to collapse and not forming a layer. The question arose, how thin one could make graphite. Two scientists from University of Manchester answered this question by peeling layers from a graphite crystal by using sticky tape and then rubbing them onto a silicon dioxide surface. They managed to isolate just one atom thick layer from graphite for the first time using a method called micromechanical cleavage or scotch tape. In this thesis chemical method also known as Hummers method has been used to fabricate graphene oxide (GO) and reduced graphene oxide. GO was synthesized through the oxidation of graphite to graphene oxide in the presence of concentrated sulphuric acid, hydrochloric acid and potassium permanganate. A strong reducing agent known as hydrazine hydrate has also been used to reduce GO to rGO by removing oxygen functional groups, but unfortunately not all oxygen functional groups have been removed, that is why the final product is named rGO. GO and rGO solutions were then deposited on silicon substrates separately. Several characterization techniques in this work have been used to investigate the optical properties, the morphology, crystallography and vibrational properties of GO and rGO.

Reduced graphene oxide as outstanding candidate electrode material for supercapacitor has been investigated. This thesis includes two topics. One is that three kinds of reduced graphene oxides were prepared by hydrothermal reduction under different pH conditions. The pH values were found to have great influence on the reduction of graphene oxides. Acidic and neutral media yielded reduced graphene oxides with more oxygen-functional groups, lower specific surface areas but broader pore size distributions than those in basic medium. Variations induced by the pH changes resulted in great differences in the supercapacitor performance. The graphene produced in the basic solution presented mainly electric double layer behavior with specific capacitance of 185 F/g, while the other two showed additional pseudocapacitance behavior with specific capacitance of 225 F/g (acidic) and 230 F/g (neutral), all at a constant current density of 1A/g. The other one is that different reduced graphene oxides were prepared via solution based hydrazine reduction, low temperature thermal reduction, and hydrothermal reduction. The as- prepared samples were then investigated by UV-vis spectroscopy, X-ray diffraction, Raman spectroscopy, and Scanning electron microscope. The supercapacitor performances were also studied and the hydrothermally reduced graphene oxide exhibited the highest specific capacitance.

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Made available in DSpace on 2016-03-15T19:36:55Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-08-18 / Fundo Mackenzie de Pesquisa / A two-dimensional structure with a thickness of one atom in a Bravais lattice in a "honeycomb". So it is known graphene, with interesting property both in basal plane as the edges of the material. Consisting of carbon atoms in its sp2 hybridization, graphene has applications to mechanical areas, electrical, optical, chemistry, biology. This requires mastered the techniques of obtaining graphene. This paper presents the study of obtaining exfoliated material, seeking to control the process as well as obtain graphene by electrochemical. The process of obtaining exfoliation is important because through it can obtain more control over the exfoliated material. The material obtained was analyzed by Raman spectroscopy. Our results show that it is possible to obtain exfoliated material, and as the spectra material by analyzing the intensity of the peaks. / Uma estrutura bidimensional com a espessura de um átomo, em uma rede de bravais em forma de favos de mel . Assim é conhecido o grafeno, com propriedade interessante tanto no plano basal como nas bordas do material. Constituído de átomos de carbono em sua hibridização sp2, o grafeno possui aplicações às áreas de mecânica, elétrica, ótica, química, biologia. Para isso é necessário dominar as técnicas de obtenção de grafeno. Este trabalho apresenta o estudo da obtenção de material esfoliado, buscando controle no processo, além de se obter grafeno por via eletroquímica. O processo de obtenção por esfoliação eletroquímica é importante, pois por meio dele pode-se adquirir mais controle sobre o material esfoliado. O material obtido foi analisado através da técnica de Espectroscopia Raman. Os resultados mostraram que é possível obter material esfoliado e a análise deste material através da intensidade das bandas de seu espectro Raman.

Graphene sheets, one atom thick, two dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes - high electronic, thermal conductivities and exceptional mechanical strength. Chemical reduction of graphene oxide (GO) has been considered as a viable route for large scale production of graphene sheets. The reduced graphene oxide (r -GO) sheets although their conductivities are comparatively lower than that of graphene are nevertheless versatile material for applications in thin films and composites. An important consideration in the design of solution processing techniques for the preparation of processable graphene sheets is the dispersibility of GO and r -GO in different solvents, especially aqueous dispersibility. While GO is dispersible in water over a wide range of pH values, r -GO shows poor dispersibility and over a limited range of pH values. Graphene oxide, the oxidized form of graphene, are single atomic layers with lateral dimension that can extend to hundreds of nanometers. The sheets contain a sizable fraction of carbons that are sp3 hybridized and covalently bonded to oxygen in the form of epoxy, carbonyls as well as ionizable hydroxyl and carboxylic functional groups located on the rim of the sheets. The remaining carbons form isolated sp2 graphene like networks. On reduction the oxygen functionalities are removed and the sp2 network partially restored. This thesis focuses on the aqueous dispersibility of GO and r -GO, and describes a strategy to enhance the dispersibility of r -GO by cyclodextrin functionalization. Chapter 1 of the thesis provides a brief review of the synthetic procedures and structure of GO and r -GO while Chapter 2 describes the experimental methods and characterization techniques used in the thesis. The chemistry underlying the aqueous dispersibility of GO and r -GO at different values of pH have been investigated by zeta potential measurements, pH titrations and infrared spectroscopy (Chapter 3). These measurements show that r -GO sheets have ionizable groups with a single pKa value (8.0) while GO sheets have groups that are more acidic (pKa = 4.3), in addition to groups with pKa values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r -GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior water dispersibility. Till recently GO was primarily considered only as an easily available precursor for chemical routes to r -GO but it has now been recognized as an interesting material in its own right. Two such attributes that have attracted wide spread attention are the in- trinsic and tunable fluorescence of GO as well as formation of liquid crystalline phases. Aqueous dispersions of GO exhibit strong pH dependent fluorescence in the visible region that originates, in part, from the oxygenated functionalities present. In Chapter 4, the spectral migration on nanosecond timescales of the pH dependent features in the fluores- cence spectra of GO is described. The changes in the steady state fluorescence spectra with pH have been correlated with the sequence of dissociation events that occur in GO dispersions at different values of pH described in Chapter 3, from time resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, it is shown that the migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfers involving the quasimolecular oxygenated aromatic fragments are observed. Aqueous dispersions of GO constitute a distinctive class of 2D-anisotropic colloids with competing interactions - long range electrostatic repulsion, originating from ionized carboxylic groups located on the rim of the sheets and weak dispersive attractive interactions originating from the un-oxidized sp2 graphitic domains. In Chapter 5, it is shown that, colloidal dispersions of GO are intrinsically frustrated, exhibiting a range of arrested or metastable states, encompassing fluid, glass and gels that coexist with liquid crystalline order. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces by changing the ionic strength of the medium, by addition of salt and/or the concentration of the dispersion. At low salt concentrations, where long range electrostatic repulsions dominates, the formation of a repulsive Wigner glass is observed while at high salt concentrations, when attractive forces dominate, the formation of gels that exhibits a nematic to columnar liquid crystalline transition. These studies highlights how the chemical structure of GO - hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet - gives rise to a rich and complex phase diagram. The poor dispersibility of r -GO in aqueous media limits its use in practical applica- tions. To enhance the dispersibility, r -GO sheets have been functionalized by covalently linking -cyclodextrin ( -CD) cavities to the sheets via an amide linkage (Chapter 6). The functionalized -CD: rGO sheets, in contrast to r -GO, are dispersible over a wide range of pH values (2 - 13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. It is shown that planar aromatic molecules adsorbed on the r -GO sheet as well as nonplanar molecules included in the tethered -CD cavities have their fluorescence effectively quenched by the -CD: rGO sheets. The -CD: rGO sheets combine the hydrophobicity associated with r -GO along with the hydrophobicity of the cyclodextrin cavities in a single water dispersible material. Resonance Raman spectroscopy is a powerful analytical tool for detecting and identi- fying analytes, but the associated strong fluorescence background severely limits the use of the technique. In Chapter 7, it is shown that the cyclodextrin functionalized -CD: rGO sheets, described in Chapter 6, provides a versatile platform for resonance Raman detection. Planar aromatic and dye molecules that adsorb on the r -GO graphitic domains and non-planar molecules included within the tethered -CD cavities have their fluorescence effectively quenched. Using the water dispersible -CD: rGO sheets, it is possible to record the resonance Raman spectra of adsorbed and included organic chromophores directly in aqueous media without having to extract or deposit on a substrate. The Raman signal intensities show a linear dependence with the concentration of analyte present in water. This is significant, as it allows for the identification and estimation of organic analytes present in water by resonance Raman spectroscopy.

The topic of this thesis is the study of nitrogen functionalities in nitrogen-doped reduced graphene oxide using Raman spectroscopy. Specifically, the project set out to investigate if the Raman active nitrogen-related vibrational modes of graphene can be identified via isotope labelling. Previous studies have used Raman spectroscopy to characterise nitrogen doped graphene, but none has employed the method of isotope labelling to do so. The study was conducted by producing undoped, nitrogen-doped and nitrogen-15-doped reduced graphene oxide and comparing the differences in the first-order Raman spectrum of the samples. Results of this study are inconclusive. However, some indications linking the I band to nitrogen functionalities are found. Also, a hypothetical Raman band denoted I* possibly related to \spt{3} hybridised carbon is introduced in the same spectral area as I. This indication of a separation of the I band into two bands, each dependent on one of these factors could bring clarity to this poorly understood spectral area. As the results of this study are highly speculative, further research is needed to confirm them and the work presented here serves as a preliminary investigation.

The solid acid catalyst of the Keggin-type 12-tungstophosphoric acid (H3PW12O40, HPW) is supported on partially reduced graphene oxide (PRGO) nanosheets for acid-catalyzed reactions. HPW is a new class of catalyst with a good thermal stability and high Bronsted acidity in order to replace common mineral acids. However, it has low specific surface area (1-5 m2/g). Therefore, the possibility of PRGO as a catalytic support for HPW is investigated due to its high surface area (2630 m2/g) and good thermal stability. The synthesis of HPW-GO catalyst is prepared using microwave and laser irradiation without using any chemical reducing agents. The HPW-GO catalysts are characterized by Ultraviolet-visible spectroscopy (UV-Vis), Fourier Transform Infrared Spectroscopy (FT-IR), Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) techniques, and Transmission Electron Microscopy (TEM). Also, the surface acidity is measured by a non-aqueous titration of n-butyl amine. Furthermore, the application for catalysts is tested by three acid-catalyzed reactions: Esterification, Friedel-Crafts acylation, and Pechmann condensation. The greatest acidity for the microwave irradiation method is with the loading of 85 wt% HPW-GO and 60wt% HPW-GO for laser irradiation. The results observed provide an excellent opportunity for PRGO as a catalytic support for HPW for acid-catalyzed reactions.

Zinc Oxide (ZnO) is one of the most extensively studied semiconductors because of its unique properties, namely, its wide band gap (3.37 eV) and high excitation binding energy (60 meV). These properties make ZnO a promising material for uses in a broad range of applications including sensors, catalysis and optoelectronic devices. The presented research covers a broad spectrum of these interesting nanomaterials, from their synthesis and characterization to their use as photocatalyts. A new synthetic approach for producing morphology controlled ZnO nanostructures was developed using microwave irradiation (MWI). The rapid decomposition of zinc acetate in the presence of a mixture of oleic acid (OAC) and oleylamine (OAM) results in the formation of hexagonal ZnO nanopyramids and ZnO rods of varying aspect ratios. The factors that influence the morphology of these ZnO nanostructures were investigated. Using ligand exchange, the ZnO nanostructures can be dispersed in aqueous medium, thus allowing their use as photocatalysts for the degradation of malachite green dye in water. Photocatalytic activity is studied as a function of morphology; and, the ZnO nanorods show enhanced photocatalytic activity for the degradation of the dye compared to hexagonal ZnO nanopyramids. After demonstrating the catalytic activity of these ZnO nanostructures, various ways to enhance photocatalytic activity were studied by modification of this MWI method. Photocatalytic activity is enhanced through band gap modulation and the reduction of electron-hole recombination. Several approaches were studied, which included the incorporation of Au nanoparticles, N-doping of ZnO, supporting ZnO nanostructures on reduced graphene oxide (RGO), and supporting N-doped ZnO on N-doped RGO. ZnO-based nanostructures were studied systematically through the entire process from synthesis and characterization to their use as photocatalysis. This allows for a thorough understanding of the parameters that impact these processes and their unique photocatalytic properties.