Master thesis “Properties of graphenoid layers” deals with materials of graphenoid nature such as graphene, graphene oxide and its reduced state. The paper effectively summarize basic theoretical knowledge in the first half of its range. In the second half the project deals with practical part consisted of experiments with application of graphene oxide solvents, its analysis and especially comparing properties of non-reduced graphene oxide with its chemically reduced form. Material is examined in a form of applied thin layers on different substrates.

Master of Science / Department of Chemistry / Duy H. Hua / Graphene oxide (GO) has attracted many researchers in the past years because of its unique electrical and chemical properties which showed the potential applications in many fields such as electronic materials and biology. Increasing research efforts in the biomedical field are bringing to light new discoveries in areas such as drug delivery, treatment of cancers, and biosensors, and are therefore attractive. The purpose of this work is to prepare GO and modify the surface of GO in order to achieve a new functionalized GO for biosensor applications in the future. GO was synthesized from the flake graphite by using a modified Hummer’s method to achieve higher quality and yield. The flake graphite was first exfoliated by using a microwave reactor. The exfoliated flake graphite then was oxidized by K[subscript]2S[subscript]2O[subscript]8, P[subscript]2O[subscript]5, and KMnO[subscript]4 under acidic conditions, followed by H[subscript]2O[subscript]2 to form GO. The following steps were to attach carboxylic acid and benzoic acid groups onto the surface of GO. Atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy were used to identity the modified GO and determine the sizes of the materials after a sequence of reactions. The modified GO will be used in the study of electronic sensing of biomolecules in Hua’s laboratory.

Magister Scientiae (Medical Bioscience) - MSc(MBS) / Graphene oxide (GO) is a single carbon layer, oxygen bearing graphene derivative, containing hydroxyl and carboxyl groups. Graphene oxide nanoparticles (GONPs) are promising nanomaterials for a variety of applications such as electrochemical analysis, adsorption of biomolecules, biosensors and drug and vaccine delivery systems. While these newly engineered nanoparticles hold great potential for developments in industry and medicine, the widespread use of these material will inevitably result in GO residues in the environment where they could possibly pose a risk to human and wildlife health. Interaction of the nanoparticles and biota can affect numerous biological processes. In humans they can affect any of the physiological systems such as the immune, endocrine, reproductive and cardiovascular systems. Although studies have indicated that GO exposure cause increased reactive oxygen species in cells, they mechanisms whereby GO act on the cell are still poorly understood. A few studies have investigated the effects of GONP and other graphene nanoparticle derivatives on the immune system. The aim of this study was to investigate the in vitro effects of GONPs on the immune system by the exposure of the murine macrophage cell line, RAW 264.7, to different concentrations of GONPs.

Neste estudo foi investigado o processo de redução térmica do óxido de grafeno em médio vácuo como uma rota viável de baixo custo econômico para obtenção do óxido de grafeno reduzido para aplicação em supercapacitores. O objetivo principal foi estudar a influência da temperatura de processamento no grau de redução do óxido de grafeno utilizando um sistema de vácuo com bomba mecânica de duplo estágio. O processamento constituiu na exposição do óxido de grafeno em várias temperaturas (200, 400, 600, 800 e 1000 °C) com pressão reduzida (10-2mbar) condição de médio vácuo. Foram utilizadas técnicas convencionais para caracterização dos materiais precursores e processados, tais como: microscopia eletrônica de varredura (MEV), difração de raios-X (DRX) e espectroscopia no infravermelho com transformada de Fourier (FTIR). Com os resultados deste estudo foi demostrado que é possível obter o óxido de grafeno reduzido utilizando um sistema de vácuo com bomba mecânica de duplo estágio e temperaturas de processamento superiores a 200°C. / In this study the process of medium vacuum thermal reduction of the graphene oxide as a low cost route for obtaining reduced graphene oxide has been investigated. The main objective was to study the influence of the processing temperature on the degree of reduction of the graphene oxide using a vacuum system with two stage backing pump. The processing was carried out by exposing the graphene oxide at various temperatures (200, 400, 600, 800 e 1000 °C) with reduced pressure (10-2 mbar). Conventional techniques have been employed to the characterization of the starting and processed materials, such as: scanning electron microscopy (SEM), X-ray diffraction and Fourier transformed infrared spectroscopy (FTIR). With the results of this study it has been demonstrated that it is possible to obtain the reduced graphene oxide using a vacuum system with a two stage backing pump and processing temperatures superior to 200°C.

Since the isolation of graphene in 2004, a large amount of research has been directed at 2D materials and their applications due to their unique characteristics. This thesis delivers pioneering developments on the applications of graphene and graphene oxide (GO) on electromagnetic ranges such as radio frequency, microwave frequency and THz bands, and specifically 2D materials based antennas, absorbers, sensors and etc. This thesis focuses on exploring electromagnetic applications of monolayer graphene, printed graphene and graphene oxide. In study of monolayer graphene applications, the theoretical and simulation studies are carried out to design tunable terahertz (THz) absorbers, tunable microwave wideband absorbers, and reconfigurable antennas, etc. These studies on the applications of monolayer graphene have proved prospective potentials of graphene in THz sensing, RCS reduction, and reconfigurable antennas. This thesis also presents pioneering advances on electromagnetic applications of printed graphene. Among these works, low-cost highly conductive and mechanically flexible printed graphene is developed for radio frequency (RF) applications. For the first time, effective RF radiation of printed graphene is experimentally demonstrated. Based on these results, applications of printed graphene including RFID (radio frequency identification) tags, anti-tampering RFID, EMI shielding, flexible microwave components such as transmission lines, resonators and antennas, conformable wideband radar absorbers, graphene oxide based wireless sensors, etc. are developed and experimentally demonstrated. This work significantly expands applications of graphene in electromagnetic areas.

>Magister Scientiae - MSc / This project focuses on the simple, fast and highly sensitive adsorptive stripping voltammetry detection of Nickel and Cobalt complexed with DMG and Nioxime respectively at a Reduced Graphene Oxide modified pencil graphite electrode in water samples. This research as well demonstrates a novel electrochemically reduced graphene oxide (ERGO)/mercury film (MF) nanocomposite modified PGE, prepared through successive electrochemical reduction of graphene oxide (GO) sheets and in-situ plated mercury film. The GO and graphene were characterized using FT-IR, HR-SEM, HR-TEM, XRD and Raman spectroscopy. The FT-IR results supported by Xray diffraction analysis confirmed the inclusion of oxygen moieties within the graphitic structure during the chemical oxidation step. Microscopic and spectroscopic analysis was used to confirm the stackings of graphene on the pencil electrode. The ERGO-PG-MFE, in combination with a complexing agents of [dimethylglyoxime (DMG) and Nioxime] and square-wave cathodic stripping voltammetry (SW-CSV), was evaluated towards the individual determination of Ni2+ and Co2+ respectively and simultaneous determination of both metals from the combination of DMG and Nioxime mixture. A single-step electrode pre-concentration approach was employed for the in-situ Hg-film electroplating, metal-chelate complex formation and its non-electrolytic adsorption at – 0.7 V for the individual analysis of Ni2+ and Co2+. The current response due to metal-ligand(s) complex reduction were studied as a function of experimental variables; deposition/accumulation potential, deposition/accumulation time, rotation speed, frequency and amplitude and carefully optimized for the individual determination of Ni2+and Co2+ and simultaneous determination of Ni2+ and Co2+ at low concentration levels (μg L-1) in 0.1 M NH3- NH4Cl buffer solution (pH 9.4) solution. The recorded limit of detection for the individual analysis of Ni2+and Co2+ was found to be 0.120 μg L-1 and 0.220 μg L-1 respectively, at an accumulation time of 120 s for both metals. The recorded limit of detection of the simultaneous analysis of Ni2+ and Co2+ was found to be 6.1 μg L-1 and 1.8 μg L-1 respectively. The ERGO-PG-MFE further demonstrated a highly selective stripping response toward all trace metal analysis. The testing of the applicability of graphene-based sensor and method in laboratory tap water samples was evaluated. This electrode was found to be sensitive enough to detect metal ions in the tap water samples at the 0.2 μg L-1 level for individual analysis and 0.001 μg L-1 for simultaneous, well below WHO standards.

Thesis (M.Sc. (Chemistry)) -- University of Limpopo, 2019. / The concept of sustainable energy development is one of the crucial topics of the 21st century. It has evolved into a guiding principle for a liveable future world where human needs are met while maintaining balance with the environment. In this regard, hydrogen technology is a promising alternative energy source since it does not produce undesirable greenhouse gas (CO2). In order to place hydrogen energy into practical applications, there are certain problems that need to be addressed, these include the efficient production and storage of hydrogen. Currently, hydrogen is mostly produced from conventional processes such as steam reforming of fossil fuels, gasification and water splitting (photo/electrochemical and thermochemical). Among these methods, electrochemical water splitting is identified as a noble process to produce clean hydrogen gas and monitor all processes through hydrogen evolution reactions (HER). The entire HER processes are sluggish in nature and cathodic electrocatalysts are utilised to accelerate the process. Hence, in this work, we present highly active graphene oxide/metal organic framework (GO/MOF) and palladium (Pd) supported GO/MOF electrocatalysts for HER. GO/MOF was prepared through impregnation method of MOF and GO, whereas Pd@GO/MOF composite was synthesised using electroless Pd deposition on GO and followed by impregnation method of direct mixing of Pd@GO and MOF. The structural, morphological and electrochemical properties of the synthesised materials (GO/MOF and Pd@GO/MOF) were characterised by X-ray diffraction (XRD), Fourier transform infrared (FTIR), simultaneous thermogravimetric analysis (STA), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), high resolution transmission electron microscopy/Energy dispersive x-ray spectroscopy/selected area electron diffraction (HRTEM/EDX/SAED) and cyclic voltammetry (CV). XRD, FTIR, TGA and DSC results revealed the presence of GO on MOF confirming the formation of composites. The SEM/EDS and HRTEM/EDX/SAED results confirmed the presence of octahedral structure of MOF in the Pd@GO sheet-like structure, elemental composition and crystallinity of the synthesised materials. Furthermore, the electrocatalytic efficiency of GO/MOF and Pd@GO/MOF composites on HER was studied using three important parameters (exchange current density, Tafel slope and charge transfer coefficient) calculated from Tafel analysis. The GO/MOF and Pd@GO/MOF composites showed excellent HER activity at 0.45 mol.L-1 H2SO4 withexchange current densities of 25.12 A.m-2 and 24.5 A.m-2, Tafel slopes of 116 mV/dec and 123 mV/dec, and transfer coefficients of 0.49 and 0.52, respectively. These observed results are consistent with theory, thus suggesting the Volmer reaction as the limiting mechanism at high concentration. However, at low concentration both composites showed an increase in the Tafel slope and transfer coefficient, suggesting the reaction order of Volmer reaction coupled with either Heyrovsky or Tafel reaction. The proposed reaction order was further supported by slope of logarithm of current as a function of pH and Pourbaix diagram. The composites demonstrated the enhancement turnover frequency (TOF) values in this order MOF <GO/MOF <Pd@GO/MOF. The large TOF value of 7.81 mol H2.s-1 in the case of Pd@GO/MOF was due the H2 spillover effect as a result of the presence of Pd nanoparticles. The fabricated composites displayed high activity, good stability and excellent tolerance to the crossover effect, which may be used as a promising catalyst in electrochemical hydrogen production and storage technology via hydrogen evolution reaction.

O poli(etileno-co-álcool vinílico) (EVOH) pertence à família de materiais poliméricos semicristalinos; é dotado de excelentes propriedades de barreira a gases e muito utilizado na fabricação de embalagens para alimentos e outros produtos sensíveis a certos níveis de oxigênio ou dióxido de carbono. Entretanto, o EVOH é muito higroscópico e em condições de alta umidade relativa perde a propriedade de altíssima barreira a oxigênio bem como as propriedades mecânicas. De acordo com literatura, a inclusão de cargas lamelares, como argila e óxido de grafeno, contribui para a melhora significativa das propriedades de barreira a gás e mecânicas do EVOH. Este trabalho estudou os efeitos da incorporação de nanofolhas de óxido de grafeno (GO) nas propriedades dos filmes de EVOH. O GO foi obtido pelo método de Hummer\'s modificado e posteriormente submetido à redução induzida por radiação ionizante. Foram incorporados 0,1- 0,3 % em peso de GO e GO reduzido (RGO) na matriz de EVOH via processo de extrusão, utilizando primeiramente uma extrusora dupla-rosca e após, uma mini extrusora balão de laboratório para a obtenção de filmes flexíveis de EVOH/GO e EVOH/RGO. Os filmes obtidos foram submetidos à radiação ionizante, em acelerador de elétrons de 1,5 MeV, com o objetivo de estudar a contribuição do tratamento por radiação ionizante, nas propriedades finais. As irradiações foram realizadas à temperatura ambiente, em ar, e faixa de dose de radiação 100-250 kGy. As amostras de GO e RGO foram caracterizadas por meio dos ensaios de difração de raios X (DRX), microscopia eletrônica de varredura com fonte de emissão de campo (MEV-FEG), microscopia eletrônica de transmissão (MET), espectroscopia vibracional de absorção no infravermelho com transformada de Fourier (FTIR), espectrometria Raman e termogravimetria (TG). As amostras dos filmes irradiadas e não irradiadas foram caracterizadas por meio de ensaios mecânicos de tração, DRX, MEV-FEG, FTIR, TG, calorimetria exploratória diferencial (DSC), taxa de permeabilidade ao oxigênio (TPO2) e espectroscopia de aniquilação de pósitrons (PALS). Os resultados dos ensaios das amostras de GO e RGO indicaram que o GO obtido pelo método de Hummer\'s modificado foi reduzido por irradiação de raios gama, como a redução dos grupos funcionais contendo oxigênio, grupos epóxi e carboxílicos. As imagens de MEV-FEG das amostras de RGO mostraram folhas separadas sem regiões dobradas e domínios agregados, já o GO apresentou uma superfície com rugosidade e empilhamento de folhas. Os filmes flexíveis de EVOH contendo GO (EVOH/GO) e RGO (EVOH/RGO) apresentaram boa dispersão do GO na matriz de EVOH. Os filmes preparados com RGO (EVOH/RGO) e submetidos à radiação ionizante apresentaram uma dispersão mais homogênea do RGO na matriz e maior adesão interfacial matriz/RGO, e, consequentemente, propriedades mecânicas superiores àquelas obtidas para os filmes de EVOH puro ou de EVOH/GO. / Poly(ethylene-co-vinyl alcohol) (EVOH) belongs to the family of semicrystalline polymeric materials; is endowed with excellent gas barrier properties, it is much used in the research area for food packaging and other products sensitive to certain levels of oxygen or carbon dioxide. However, EVOH is very hygroscopic and the high flow conditions relative to the very high barrier property are oxygen as well as the mechanical properties. According to the literature, an inclusion of lamellar loads, such as clay and graphene oxide, contribute to the strengthening of the gas properties and the mechanical discharges of EVOH. This work was studied the addition graphene oxide (GO) nanosheets into EVOH properties. The GO was obtained by the modified Hummer\'s method and subsequently submitted to the reduction induced by ionizing radiation, 0.1-0.3 % by weight of GO and reduced GO (RGO) were incorporated into the EVOH matrix via the extrusion process, using firstly a double-screw extruder and then a mini-laboratory extruder for obtaining films EVOH / GO and EVOH / RGO. The obtained films were submitted to ionizing radiation, in an electron accelerator of 1,5 MeV, in order to study the contribution of the treatment by ionizing radiation, in the final properties. The irradiations were performed at room temperature in air, and radiation dose range 100-250 kGy. The GO and RGO samples were characterized by X-ray diffraction (XRD), scanning electron microscopy with field emission source (SEM-FEG), transmission electron microscopy (TEM), vibration absorption spectroscopy in the Fourier transform infrared (FTIR), Raman spectrometry and thermogravimetric (TG). The irradiated and non-irradiated films were characterized by mechanical tests, XRD, SEM-FEG, FTIR, TG, differential scanning calorimetry (DSC), oxygen permeability rate (TPO2) and positron annihilation spectroscopy (PALS). Tests results of GO and RGO samples indicated that the GO obtained by the modified Hummer\'s method was reduced by gamma irradiation, such as the reduction of the oxygen-containing functional groups, epoxy groups and carboxylic groups. The SEM-FEG images of RGO samples showed separate leaves without folded regions and aggregate domains, whereas the GO showed a surface with roughness and stacking of leaves. EVOH flexible films containing GO (EVOH / GO) and RGO (EVOH / RGO) showed good dispersion of GO in the EVOH matrix. The films prepared with RGO (EVOH / RGO) and subjected to ionizing radiation presented a more homogeneous dispersion of RGO in the matrix and higher interfacial matrix / RGO adhesion, and, consequently, superior mechanical properties to those obtained for pure EVOH or EVOH/GO films.

Many diseases have been associated with oxidative stress (OS) which is caused when the production of reactive oxygen species (ROS), such as superoxide (O2•-) and hydroxyl radical (•OH), overcome the scavenging efficiency of living organisms. It is known that ROS production is worsened during traumas related to ischemic events and subsequent reperfusion in which the treatment with fast and effective antioxidants is critical to prevent cell and tissue damage. PEG-HCCs are carbon nanoparticles that showed O2•- and •OH scavenging properties according to electron paramagnetic resonance (EPR) experiments and peroxyl scavenging properties based on oxygen radical absorbance capacity (ORAC) assays. The O2•- quenching capability was also examined in vivo using a mild traumatic brain injury (mTBI) model complicated with hypotension. As result of the PEG-HCCs treatment, the cerebral blood flow (CBF) was restored while normalizing O2•- and nitric oxide (NO•) levels, primarily in the cerebral vasculature

Graphite oxide (GO), a carbon material prepared in one step from low cost commercial materials, and graphene oxide have been found to catalyze a wide range of reactions including oxidations, hydrations, and dehydrations, as well as cationic or oxidative polymerizations. Applicable in both small molecule and polymer chemistry, this single, metal-free catalyst shows remarkable breadth, including the combination of the aforementioned reactions in an auto-tandem fashion to form advanced substrates, such as chalcones, from simple starting materials. Some of these reactions, such as the selective oxidation of alcohols to aldehydes, have been shown to be dependent on the presence of molecular oxygen, suggesting that this may be the terminal oxidant. Aside from its eminently valuable reactivity, the use of GO as a catalyst also presents practical advantages, such as its heterogeneous nature, which facilitates separation of the catalyst from the desired product. The use of this simple material in synthetic chemistry, as well as others like it, is distinct from other forms of catalysis in that the active species is carbon-based, heterogeneous and metal-free (as confirmed by ICP-MS and other spectroscopic techniques). This has led us to propose the term “carbocatalyst” to describe such materials. With dwindling supplies of precious metals used in many common organic reactions, the use of inexpensive and widely available carbocatalysts in their place will ensure that commercial processes of fundamental importance can continue unabated. Moreover, as we have shown with just one material, carbons are capable of facilitating a broad range of reactions. / text