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Browsing Faculty of Science and Technology by Author "Bello, A."
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Item Chemical adsorption of NiO nanostructures on nickel foam-graphene for supercapacitor applications(J. Mater Sci (2013), 2013) Bello, A.; Fabiane, M.; Dodoo-Arhin, D. et alFew-layer graphene was synthesized on a nickel foam template by chemical vapor deposition. The resulting three-dimensional (3D) graphene was loaded with nickel oxide nanostructures using the successive ionic layer adsorption and reaction technique. The composites were characterized and investigated as electrode material for supercapacitors. Raman spectroscopy measurements on the sample revealed that the 3D graphene consisted of mostly few layers, while X-ray diffractometry and scanning electron microscopy revealed the presence of nickel oxide. The electrochemical properties were investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and potentiostatic charge�discharge in aqueous KOH electrolyte. The novelty of this study is the use of the 3D porous cell structure of the nickel foam which allows for the growth of highly conductive graphene and subsequently provides support for uniform adsorption of the NiO onto the graphene. The NF-G/NiO electrode material showed excellent properties as a pseudocapacitive device with a high-specific capacitance value of 783 F g-1 at a scan rate of 2 mV s-1. The device also exhibited excellent cyclestability, with 84 % retention of the initial capacitance after 1000 cycles. The results demonstrate that composites madeusing 3D graphene are versatile and show considerable promise as electrode materials for supercapacitor applications.Item A dilute Cu(Ni) alloy for synthesis of large-area Bernal stacked bilayer graphene using atmospheric pressure chemical vapour deposition(AIP Publishing, 2016-01-07) Fabiane, M.; Madito, M. J.; Bello, A.; Oliphant, C. J.; Dangbegnon, J. K.; Jordaan, W. A.; Momodu, D. Y.; Masikhwa, T. M.; Barzegar, F.; Manyala, N.A bilayer graphene film obtained on copper (Cu) foil is known to have a significant fraction of non-Bernal (AB) stacking and on copper/nickel (Cu/Ni) thin films is known to grow over a large-area with AB stacking. In this study, annealed Cu foils for graphene growth were doped with small concentrations of Ni to obtain dilute Cu(Ni) alloys in which the hydrocarbon decomposition rate of Cu will be enhanced by Ni during synthesis of large-area AB-stacked bilayer graphene using atmospheric pressure chemical vapour deposition. The Ni doped concentration and the Ni homogeneous distribution in Cu foil were confirmed with inductively coupled plasma optical emission spectrometry and proton-induced X-ray emission. An electron backscatter diffraction map showed that Cu foils have a single (001) surface orientation which leads to a uniform growth rate on Cu surface in early stages of graphene growth and also leads to a uniform Ni surface concentration distribution through segregation kinetics. The increase in Ni surface concentration in foils was investigated with time-of-flight secondary ion mass spectrometry. The quality of graphene, the number of graphene layers, and the layers stacking order in synthesized bilayer graphene films were confirmed by Raman and electron diffraction measurements. A four point probe station was used to measure the sheet resistance of graphene films. As compared to Cu foil, the prepared dilute Cu(Ni) alloy demonstrated the good capability of growing large-area AB-stacked bilayer graphene film by increasing Ni content in Cu surface layer.Item Graphene: Synthesis, Transfer, and Characterization for Dye-Sensitized Solar Cells Applications(ACS Publications, 2013) Dodoo-Ahrin, D.; Fabiane, M.; Bello, A.Large area graphene films with varying growth time thicknesses were synthesized by chemical vapor deposition (CVD) on copper foil and transferred to other substrates for characterization and photovoltaic applications. Scanning tunneling microscopy, Raman spectroscopy, and imaging show films with high crystallinity, low defect density, I2D/IG ratio of 0.52?2.26, and the presence of continuous graphene sheets with optical transparency of 75.7%?96.8%. The use of CVD graphene sheets as counter electrode in dye-sensitized solar cells was explored. The short circuit current density (Jsc), the open circuit current (Voc), the fill factor (FF), and the overall conversion efficiency under AM 1.5, 100 mW cm?2 illumination are 12.7 mA/cm2 57.5% and 3.8%, respectively.Item Growth of graphene underlayers by chemical vapor deposition(AIP Publishing, 2013) Fabiane, M.; Khamlich, S.; Bello, A.We present a simple and very convincing approach to visualizing that subsequent layers of graphene grow between the existing monolayer graphene and the copper catalyst in chemical vapor deposition (CVD). Graphene samples were grown by CVD and then transferred onto glass substrates by the bubbling method in two ways, either direct-transfer (DT) to yield poly (methyl methacrylate) (PMMA)/graphene/glass or (2) inverted transfer (IT) to yield graphene/PMMA/glass. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were used to reveal surface features for both the DT and IT samples. The results from FE-SEM and AFM topographic analyses of the surfaces revealed the underlayer growth of subsequent layers. The subsequent layers in the IT samples are visualized as 3D structures, where the smaller graphene layers lie above the larger layers stacked in a concentric manner. The results support the formation of the so-called �inverted wedding cake�stacking in multilayer graphene growth.Item High-performance symmetric electrochemical capacitor based on graphene foam and nanostructured manganese oxide(AIP Publishing, 2013) Bello, A.; Fashedemi, O. O.; Lekitima, J. N. et alWe have fabricated a symmetric electrochemical capacitor with high energy and power densities based on a composite of graphene foam (GF) with ?80 wt% of manganese oxide (MnO2) deposited by hydrothermal synthesis. Raman spectroscopy and X-ray diffraction measurements showed the presence of nanocrystalline MnO2 on the GF, while scanning and transmission electron microscopies showed needle-like manganese oxide coated and anchored onto the surface of graphene. Electrochemical measurements of the composite electrode gave a specific capacitance of 240 Fg?1 at a current density of 0.1 Ag?1 for symmetric supercapacitors using a two-electrode configuration. A maximum energy density of 8.3 Whkg?1 was obtained, with power density of 20 kWkg?1 and no capacitance loss after 1000 cycles. GF is an excellent support for pseudo-capacitive oxide materials such as MnO2, and the composite electrode provided a high energy density due to a combination of double-layer and redox capacitance mechanisms.Item Hydrothermal synthesis of simonkolleite microplatelets on nickel foam-graphene for electrochemical supercapacitors(J. Solid State Electrochem, 2013) Khamlich, S.; Fabiane, M.; Bello, A.Nickel foam-graphene (NF-G) was synthesized by chemical vapour deposition followed by facial in situ aqueous chemical growth of simonkolleite (Zn5(OH)8Cl2�H2O) under hydrothermal conditions to form NF G/simonkolleite composite. X-ray diffraction and Raman spectroscopy show the presence of simonkolleite on the NF-G, while scanning and transmission electron microscopies show simonkolleite micro-plates like structure evenly distributed on the NF-G. Electrochemical measurements of the composite electrode give a specific capacitance of 350 Fg?1 at current density of 0.7 Ag?1 for our device measured in three-electrode configuration. The composite also shows a rate capability of ~87 % capacitance retention at a high current density of 5 Ag?1 as an electrode material for supercapacitor applications, which makes it a promising candidateItem Microwave assisted synthesis of MnO2 on nickel foam-graphene for electrochemical capacitor(Elsevier, 2013) Bello, A.; Fashedemi, O. O.; Lekitima, J. N. et alA green chemistry approach (hydrothermal microwave irradiation) has been used to deposit manganese oxide on nickel foam-graphene. The 3D graphene was synthesized using nickel foam template by chemical vapor deposition (CVD) technique. Raman spectroscopy, X-ray diffraction (XRD), scanning electron and transmission electron microscopies (SEM and TEM) have been used to characterize structure and surface morphology of the composite, respectively. The Raman spectroscopy measurements on the samples reveal that 3D graphene consists of mostly few layers with low defect density. The composite was tested in a three electrode configuration for electrochemical capacitor, and exhibited a specific capacitance of 305 F g?1 at a current density of 1Ag?1 and showed excellent cycling stability. The obtained results demonstrate that microwave irradiation technique could be a promising approach to synthesis graphene based functional materials for electrochemical applications.Item Raman spectroscopy and imaging of Bernalstacked bilayer graphene synthesized on copper foil by chemical vapour deposition: growth dependence on temperature(2017) Fabiane, M.; Madito, M. J.; Bello, A.; Manyala, N.We report on the effect of temperature on the growth of bilayer graphene on a copper foil under atmospheric pressure chemical vapour deposition (AP-CVD). Before characterization of the AP-CVD bilayer graphene, a high-quality graphene flake was obtained from the Kish bulk graphite by micro-mechanical exfoliation and characterized by using Raman spectroscopy and imaging. The Raman data of the exfoliated, high-quality graphene flake show monolayer and bilayer graphenes and were compared with the Raman data of AP-CVD graphene. Raman spectroscopy of AP-CVD graphene shows bilayer films that exhibit predominantly Bernal stacking with an I2D/IG ratio of ~1. At low growth temperature (~780 °C), Raman disorder-related peak intensity in the AP-CVD graphene is high and decreases with an increase in growth temperature to the lowest disorder intensity at ~973 °C. The selected area electron diffraction and atomic force microscopy average step height analysis showed the thickness of the bilayer graphene. The AP-CVD graphene is uniform at low growth temperatures (~780 °C) with a high disorder and becomes nonuniform at high growth temperatures (~867–973 °C) with a very low disorder as bilayer graphene evolves to form islands with an average lateral size of <10 μm. Competition between carbon adatoms supply through dehydrogenation of the CHx species, mobility and desorption rate of the carbon-adatom species for nucleation of the bilayer graphene as a function of temperature is elucidated. This study provides further insight into the growth mechanisms of bilayer graphene by AP-CVD on Cu.Item Silver nanoparticles decorated on a three-dimensional graphene scaffold for electrochemical applications(Journal of Physics and Chemistry of Solids, 2013) Bello, A.; Fabiane, M.; Dodoo-Arhin, D. et alSilver metal nanoparticles were decorated by electron beam evaporation on graphene foam (GF) grown by chemical vapour deposition. X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy, and atomic force microscopy were used to investigate the structure and morphology of the graphene foam/silver nanoparticles (GF/Ag). Both samples were tested as electrodes for supercapacitors. The GF/Ag exhibited a significantly higher capacitive performance, including a specific capacitance value of (110 Fg1) and excellent cyclability in a three-electrode electrochemical cell. These results demonstrate that graphene foam could be an excellent platform for metal particles for investigating improved electrochemical performance.